Adding mouse support for SDL builds. Currently used for:
Look around:
* Left click to move cursor

Targeting:
* Left click to move cursor
* Scrollwheel to cycle targets
* Right click to fire
Use input_event struct to encapsulate mouse events
Fix 'if' block missing braces and const correctness
Add a return statement to dummy getch_kyb_mouse definition in wincurses, in case that's causing some compilers to complain.
Move input_event into a common input definitions header that can be shared with Cygwin and Linux build configurations.
Merge basic mouse support.
Final build fix for mouse support.