Added Window#on_click, Window#on_right_click and Window#on_move

These are event handlers that we now can use in Ruby to augment /
interact with a running application.
Move get function generators to their respective headers

This is due to two reasons:

1) It keeps everything coherent
2) If we need to define custom holder types (see window.h and
window.cc), it just makes things a lot easier.
Merge pull request #4 from andremedeiros/mouse-callbacks

Added mouse callbacks to allow interacting with a running application.