kcms/tablet: Overhaul button binding, move into dialog

Instead of solely allowing keystroke bindings, this changes the binding process in the KCM drastically. It now pops up in a dialog, allowing a user to choose between using keyboard bindings, mouse click bindings or an application defined action.

WIP screenshot:

TODO

• Add support to KWin for mouse bindings with keyboard mods (currently it's only the mouse buttons themselves AFAIK)
  • See kwin!6095 (merged) (ready to merge)
• Add support to KWin for disabled buttons
  • See kwin!6092 (merged)
• Mouse clicks don't open in the correct place
  • See kwin!6146 (merged)

Future Considerations (not blockers ATM)

• Think about how scroll up/down actions would work. This is currently a limitation in the kcminputrc format and will need implementation in KWin.
• Think about how default keybinds fit into this. Awaiting proper libwacom integration, but still needs consideration in this UX for the future.

BUG: 469232