Add support for built-in keypad navigation

[tronical]

As a follow-up to https://github.com/slint-ui/slint/discussions/5550 we could add support for the following feature to Slint:

• Add a keypad-navigation: true; boolean property to FocusScope.
• If the FocusScope receives an ArrowUp/Down/Left/Right key and keypad-navigation is true, then we could search from the root of the window through all elements that are FocusScope instances, have keypad-navigation set to true, and select the one that is nearest in the given direction.
• On the located element, invoke focus().

[Enyium]

For this, the behavior of text editors may be relevant when the end user uses vertical arrow keys. States in chronological order with | representing the cursor:

abcd|f
ab
abcdef

abcdef
ab|
abcdef

abcdef
ab
abcd|f

The text editor remembers a "theoretical" (as opposed to practical) horizontal cursor position, and, if no horizontal navigation overrode it, it derives the practical cursor position from the theoretical cursor position on every vertical navigation to a new line.

In the same way, Slint could perhaps remember the position and bounds of the element that it navigated away from, and, if another navigation action is what directly follows, this information is used to determine the most suitable next element to focus to stay on the navigation path as best as possible, even if an intermediately focused element had the potential of leading the navigation path astray.

[tronical]

Excellent idea!