Tile half screen shortcuts
Fixes #64 (closed)
Adds four shortcuts that allow you to snap a window to fill half the screen, at the left, right, top or bottom.
For example, on logging in. Super-T - new terminal window Super-Shift left - tiles it left Super-T - another new terminal window Super-Shift right - tiles it right
Now you have two terminal windows tiles beside each other on each half of the display.
Edited by Andrew Mackenzie