Unify Shortcuts: join keyboard & mouse settings

[Don Ptah]

Allow users to both see and change all shortcut behaviour in one place:
1. Unite all mouse and keyboard shortcut settings;
2. Allow more than one shortcut to be listed for each action.

As is, in Settings -> Interface you have:
1. 'shortcuts' - for keyboard only, divided into browser and viewer sets;
2. 'keyboard' - can be retained for specific behaviour, like "automatic" or "only in fullscreen", but essentially - same as above, just keyboard shortcuts;
3. 'mouse' - same. Make it possible to list more than one shortcut for an action, and there is no need for a separate tab like this;
4. 'switching' - same. Just make the commands listed here a part of a universal shortcut-and-action list. Action commands should be the keys, while shortcuts (both mouse and keyboard) should be freely appointed to those keys (save for conflicts).

Also, some mouse shortcuts work, but are unlisted and unchangeable.

This is related to my other suggestion on zoom-and-panning mouse behaviour in fullscreen, but decided to write it out separately since it's a little broader.