Keyboard handler rewrite

[Spengreb]

I rewrote how LUPPP handles the keyboard shortcuts. This pull request adds a new context menu button when right clicking on a clip. It shows the current hotkey assigned to this clip and allows the user to change it with a single button press. If another hotkey is already assigned somewhere else it will remove that assignment applying it to only the current clip in context.

Heres what it looks like:

This addresses quite a few problems from the previous implementation. One thing was that the hotkeys are not communicated in any way to the user and when first using LUPPP it was a little confusing, especially when not using a US/UK qwerty keyboard.

Hotkeys are saved to the luppp.prfs file and it now auto saves the prfs on exit.

~~As it stands in this PR the new hotkeys are only set for that instance of the application. I want this to persist from one luppp launch to the other and~~ I would like to make the other special keys configurable too in the setup menu. To prevent this branch from ballooning too much I think I'm at a point where its good enough to merge.