Add support for basic mouse functions

[Vangamar]

This would be one of the key features of the Windows-centric "Gaming Software" that is not currently on the Linux version (I'm gonna call the the ecraven/g13 github project the "Linux version"!), being able to map mouse functions to the G13 sounds like an edge case, but sometimes you need it for quirky game X, Y or Z.

There seems to be some wish to add mouse functionality via the stick, but so far ABSOLUTE doesn't seem to do anything and neither do the two thumb buttons next to the stick.

It would be really useful to have at least the mouse left, right, and center button clicks (making and breaking) as "actions" that can be bound in a lot of games. I'm not convinced of the utility of the thumbstick as a mouse per se, but being able to left-shift-left-mouse-click with a g13 key would definitely help with various e.g. WoW functionality. Or is it there but I just don't see it? The shifts are there...