Remove `Is` from the getter names for the same reason that removing `Get` from the name is preferred. The word `Is` was inconsistently used as a prefix in `CScrollRegion` but as an infix in `CListBox`.
Store the list box active state inside `CListBox` instead of tracking it separately with a pointer.
Allow activating list boxes by clicking the scrollbar. Previously it was only possible to activate list boxes by selecting an item.
Fix country selection list box in players settings not being deactivated properly when name/clan edit boxes are active, because wrong UI element IDs were being used.
Replace existing listbox implementations (`CMenus::UiDoListbox*` and `HandleListInputs` functions) with `CListBox` from upstream.
Reimplement additional feature that was already present in ddnet: page up/down, home and end key handling.
Affects the following lists:
- server browser
- server browser scoreboard
- server browser friends
- country / region selection popup (server browser filter)
- player skin list
- player country / region list
- theme list
- assets list
- graphics resolutions list
- dropdown menus (e.g. graphics fullscreen mode)
- ingame player list
- vote options list
- kick/specvote lists
- ghost list
- language list (in settings and in popup on first launch)
- demo browser
- editor file browser (saving, loading, adding images / sounds)
- The search / filename input is also improved so navigating a filtered list works correctly by porting the logic from upstream.
There are minor changes to the visual appearance of some lists, due to changed margins.
The vertical alignment of some list item texts is improved so the text is centered vertically.
