Add cause field to FocusGained event

[jordanhalase]

Objective

• Users would like the ability to know how a focus was gained

Solution

• Add FocusCause field to FocusGained event
• Automatically select all when navigating into a text input (even without SelectAllOnFocus)

Testing

• Tab into the color input of feathers_gallery to test that its text gets highlighted

[jordanhalase]

This touches on many different files, please let me know if the choices to propagate FocusCause::Pressed vs. FocusCause::Navigated were correct.

1. As it currently stands, middle-clicking or secondary-clicking will propagate a FocusCause::Navigated event, only primary clicking sends a FocusCause::Pressed. This may be intentional. EditableText::on_focus_select_all() currently relies on this being the case.

   On Discord we've decided to go the route for using FocusCause::Navigated for unknown or default cases and non-primary mouse presses. This also distinguishes its behavior for future right-click menus.

2. The default behavior for HTML inputs is to always highlight all text when tabbed/navigated into so I followed that behavior. I don't know if it's worth opting out of this with some NoSelectAllOnNavigation marker to preserve previously highlighted text when tabbing back to it. (Could be useful for larger multi-line text inputs)

   For now I am following the HTML standard of selecting all for single line inputs on navigation or retaining selection for multiline inputs

[jordanhalase]

Closes #23976

[alice-i-cecile]

Can you say more about why this is useful? I think I have an intuition, but it's helpful to write it down explicitly.

[viridia]

Looks good overall. One issue to consider is whether "initial" focus should be considered "navigated" or "pressed".

For example, when a modal dialog opens, it is common practice to set focus to some widget inside the dialog. If the dialog contains a text input field, then we will want to select all the text so that the user can immediately start typing, without having to click on it. Similar logic goes for input fields with the AutoFocus marker.

Other widgets that automatically set focus on opening are popup menus, but in that case it doesn't matter. However, for consistency, I would suggest that "navigated" be the choice for any widget that is automatically focused when it appears. (We could add a third option to the FocusCause enum, but I don't see the need and it would make the logic more complex.)