Redesign the shortcut system for activating tools.
We used QActions which have the disadvantage that they go in front of everybody, which meant that a krita shortcut without things like Ctrl would make it impossible for users to type that key in other places.
This new solution expects a tool to property 'accept' used keys and all the keys that are unaccepted are now scanned for a match with the tools keyboard shortcuts.
If there is a match, we activate the tool.

This limits the keys to activate a tool from being application-wide to only working when the canvas has focus.

I'm convinced that this is the only sensible way we can actually support one-key shortcuts, though.