Renaming the enum values to be clearer.
More renaming.
Renaming the composite modes.

Fixes #52.
Fixing header guards to be more standard.
Spelling fixes.
Adding keyboard shortcuts for different composite modes.

 Escape - No composite mode
 F1 / p - Picture-in-Picture
 F2 / d - Side-by-side (preview)
 F3 / s - Side-by-side (equal)
 Tab    - Cycle composite modes

Fixes issue #45.
Updating the README information with new shortcut table.
Removing support for older gtk versions.