[Input] Added KeyModifiers enumeration

KeyModifiers are represented as a bitwise combination of the Alt,
Control and Shift keys.
[Mac] Implemented modifier keys
[SDL2] Corrected win/alt key mapping (was swapped previously)
[SDL2] Added PeepEvents/PollEvent functions
[Input] Added modifier keys to KeyboardKeyEventArgs
[SDL2] Implemented modifier keys

Additionally, removed event watch callback in favor of calling
PollEvent() directly in ProcessEvents, which should be more efficient.
[Input] Implemented GetModifiers() method
[Win] Implemented modifier keys
[X11] Implemented modifier keys
[Examples] Display modifier key information
[X11] Fixed KeyUp event arguments

KeyUp event was incorrectly fired with KeyDownEventArgs instead of KeyUpEventArgs. This is now fixed.
Merge branch 'keymod' into develop