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.

[harryhaaren]

try {} catch {} or atoi for avoiding C++ exception throwing. Elegant solution (much more so than my "i'm learning C++ so duplicating code everywhere" approach :)

[Spengreb]

The above comments should all be resolved. I also did a bit of a refactor of the hotkey dialog box as it was a bit messy before. I fixed some bugs i noticed along the way also. Such as not allowing SHIFT and ESC to be set as a hotkey. Since SHIFT is the inverse of an action, it caused some very confusing behavior if allowed to be set as a hotkey as well.

I also changed the format of hotkey dialog on the front end to try to communicate some of this to the user here is what it looks like now: