Improve keyboard mapping UI

[jtsiomb]

Instead of having a list of a handful common keysyms, and allowing the user to type keysym names in the field, it would be much simpler to have a button which asks the user to press the key they want to map.

[TimSchimansky]

+1, that would be great. Basically making the buttons a mini macro pad

[blakeblair]

ye, especially because apps like blender do not recognize the buttons alone, being able to map Crtl_L + Alt_L + S and the like is really important.