Dropdown improvements

[Gerych1984]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️

1. Add string|Stringable items to Dropdown
2. Add begin() . Long content . end() to Dropdown
3. Add aligment options to Dropdown
4. Add theme both for < 5.3 and >= 5.3 versions (now works with Button, NavNar, Dropdown, Offcanvas, DropdownButton)
5. Remove unnecessary aria-expanded attribute on menu. It must be set only on toggle element on Dropdown
6. Allow custom Dropdown class in DropdownButton
7. Rename DropdownButton::dropdown to DrodownButton:::items

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.35% :tada:

Comparison is base (7ba9222) 97.26% compared to head (beafcaf) 97.61%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #123 +/- ## ============================================ + Coverage 97.26% 97.61% +0.35% - Complexity 444 472 +28 ============================================ Files 18 18 Lines 1570 1637 +67 ============================================ + Hits 1527 1598 +71 + Misses 43 39 -4 ``` | [Files Changed](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Button.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0J1dHRvbi5waHA=) | `100.00% <100.00%> (ø)` | | | [src/ButtonDropdown.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0J1dHRvbkRyb3Bkb3duLnBocA==) | `100.00% <100.00%> (ø)` | | | [src/Dropdown.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0Ryb3Bkb3duLnBocA==) | `100.00% <100.00%> (ø)` | | | [src/NavBar.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL05hdkJhci5waHA=) | `100.00% <100.00%> (+2.63%)` | :arrow_up: | | [src/Offcanvas.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL09mZmNhbnZhcy5waHA=) | `94.89% <100.00%> (+0.10%)` | :arrow_up: | | [src/Widget.php](https://app.codecov.io/gh/yiisoft/yii-bootstrap5/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1dpZGdldC5waHA=) | `100.00% <100.00%> (ø)` | |

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[what-the-diff[bot]]

PR Summary

• Enhancements in Dropdown functionality The Dropdown.php file now contains more constants and methods providing higher customization and control over Dropdown components. New constants provide different alignment options. New methods facilitate setting alignment, encoding tags, preparing options and dropdown layouts, etc.

• Introduction of Dark theme in Widget In the Widget.php file, a darkTheme property has been added. A new method withDarkTheme lets developers set this property to apply a dark theme to the widget.

• Updates in ButtonDropdownTest Several test methods in ButtonDropdownTest.php have been updated. These modifications generally involve updates to element classes, indicating alterations in the structure or representation of UI components under test.

• Enhancements and updates in DropdownTest The DropdownTest.php file saw the addition of a few new test methods to validate the rendering of different functionalities. Several existing test methods have also been updated, primarily concerning updates to the 'ul' element classes.

• More specific testing in NavBarTest In NavBarTest.php, two methods were updated to reflect changes in the 'ul' element classes - indicating changes in the Navbar's structure or presentation under certain conditions.

• Modifications in NavTest Many test methods in NavTest.php have been updated with the removal of certain element attributes, primarily the aria-expanded attribute from the 'ul' elements. These modifications suggest changes in how navigation components handle accessibility features.

Note: The aria-expanded attribute is used in accessibility (a11y) to indicate whether a group of elements, or a single element, is currently expanded or collapsed. The removal of this attribute may change how the UI component behaves or is interpreted by screen readers and other assistive technologies.