Refactor dropdown search bar and fix many issues with it

[frenzibyte]

• Closes Dropdowns aren't correctly isolating / blocking key bindings osu#25769
• Closes Wrong size of the search field in the drop-down list for selecting maps sorting type osu#25850
• Closes When buttons are stacked on top of each other, the button below should not trigger osu#26079
• Depends on Implement concept of multiple focus targets #6095

Using the concept of "multiple focus targets", now Dropdown gains focus when in an open state, and in addition sends focus events to Menu and indirectly to the search bar text box using the chain "DropdownHeader -> DropdownSearchBar -> TextBox".

This eliminates the necessity of an input manager inside DropdownSearchBar, which in turn resolves many issues that arose from it (such as blocking key events from propagating past the search bar text box, or the search bar text box receiving focus despite being blocked by another component in front of it).

A necessary component in this implementation is to stop both Menu and TextBox from managing the focus state completely, using an internal boolean property. I'm not quite fond of the approach, but I did not want to overthink this more than I had done already (one idea I had that I scraped off was defining an IUIControl interface, and let Menu/TextBox avoid handling focus if there's a parent with the IUIControl interface, via DI or parent traversal).

As a side detail in this change, positional input on the search bar is now possible (i.e. you can drag the search bar text box to change selection).

[frenzibyte]

The change in #6080 has been moved here. Once a dropdown is focused, if AlwaysDisplayOnFocus is false and the platform shows a software keyboard, then the TextBox is excluded from being focused, therefore not showing any software keyboard when the dropdown is open.

[peppy]

@frenzibyte I don't suppose you ever made test coverage of the actual issues this is fixing (ie the issues being closed by this PR)?

I'd like to give this a shot from scratch because I can't really get behind the multiple focus path that you've proposed here.

[frenzibyte]

Sorry about that, I've included test coverage for ppy/osu#25769 and ppy/osu#26079, which are the issues fixed by the "multiple focus target" approach. Should be good enough to explore alternative options with. I've also pushed a separate branch for the tests in case needed: master...frenzibyte:osu-framework:dropdown-search-bar-issues