Add documentation for combobox

[Keboo]

There are several additional features provided by the combo box that are not that easy to discover and should be documented.

• [x] Add a page to the Wiki for ComboBox
• [x] Add a link to it in the sidebar under Controls
• [x] Document the two possible styles for ComboBox (MaterialDesignComboBox - this is the default, and MaterialDesignFloatingHintComboBox)
• [x] Document that in addition to the default brushes on ComboBox there are a couple attached properties for changing colors. These are TextFieldAssist.UnderlineBrush for changing the color of the underline when the ComboBox is editable and the user is editing. HintAssist.Foreground for changing the color of the hint (when it is used).
• [x] Document the ComboBoxAssist.ClassicMode attached property. When this is set to true it will cause the popup with the items to display strictly outside of the ComboBox rather than around it. This is more closely aligned with the default WPF ComboBox.

[Keboo]

Related #1171

[kevinmbuthi]

I agree, making a PR soon.

[Pressacco]

Let me begin by saying that I am really impressed with this library. Thank you to all of those who contributed to the project!

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It would be helpful if the documentation provided some guidance on how to implement the WPF default behavior for an editable ComboBox:

1. DO NOT display list of items when ComboBox is clicked.
   • The user can simply click the combo box and start typing without having the dropdown obstructing the UI.
2. DO display list of items when ComboBox arrow is clicked.
   • When the user wants to see the list of options, they can explicitly request to see them by clicking the combo box arrow.

I am still relatively new to WPF, and the solution has escaped me.

Additional Context

It appears that the MaterialDesignInXaml default behavior for a IsEditable="True" ComboBox is to:

• automatically display the list of dropdown items on PreviewMouseDown or MouseLeftButtonDown

[Alex-451]

@Keboo I added a page for ComboBox Note that I did not have to create a pr (Is everyone supposed to be able to just add/edit pages without anyone actually approving the changes?)

[Keboo]

@Alex-451 yes, the wiki is just a special git repo within GitHub. I do keep an eye on the changes that people make to it and undo changes when needed, but most of the time when people make changes it is making things better, so I have left it open to all edits. I have looked over your changes and they look great to me.

[Keboo]

If you are looking to get PR credit for hacktoberfest, feel free to submit a small PR to this file adding a wiki link to the new page. You can see an example of suck a link here