Fix caps lock being considered shift lock, clean up tabbox modifiers handling and fix custom tiling not triggering with some keyboard settings
Commit 1: Caps lock is not shift lock. For some reason it was changed to be considered as shift lock in ec98f498... but that's wrong afaiu. Shift lock is a separate thing, and is handled by libxkbcommon
Commit 2: With ^ fixed, remove the workaround I introduced for the tabbox modifier handling, and use the normal modifiers for the tabbox - they're the same now anyways
Commit 3: use normal keyboard modifiers for triggering custom tiling. Pressing shift isn't really a global shortcut, and using the global shortcut logic for this breaks with some keyboard layouts and settings
For more details on the bug: With these keyboard layouts or settings, the shift modifier is consumed by the xkb state machine for triggering caps lock, if the modifier is active when a second shift key is pressed. When we check what modifiers are consumed, we check if pressing a shift key would consume the shift modifier... So that always returns true. Idk if that can actually be fixed in a more "proper" way that also works for global shortcuts, but a shift-only global shortcut would be very weird, so I'd just ignore that theoretical issue tbh.