These aren't used in many places, but we've since moved away from
opacity in UI elements like this, so let's just nuke it here for
legibility.
Addresses https://github.com/ppy/osu/discussions/29906.
It didn't ever really make sense for it to be sharing the implementation
details of that (e.g. colouring of primary/dangerous actions), and with
the hotkey display things got outright hacky, so I'm decoupling it
entirely.
I've also removed inheritance from `SearchTextBox` because it contains
logic that might interfere with the internal implementation of dropdown
search bars (focus logic and stuff).
This sort of thing has been showing up on flyte designs more and more
so I want to start using it more over that rather ugly "overlined" text
that's there on multiplayer screens right now.
Also includes a test case in `TestSceneShearedButton`s to ensure the buttons' shear factors don't change on different heights (I've encountered issues with that previously).