Introduce Actions

 Actions are event re-actions.
 Currently only ButtonPress event is handled by Actions and only
 one action is defined, which is called Spawn (run external command).
 Type (and parser) can be extended to EWMH actions (switch to desktop,
 close window, whatever).
Use fmap instead of >>=
Fix actions for right side of the bar

 totSLen was wrong
Help type inferencer a bit, kills warning
Shorter lines
Add action example
Better action example explanation
Remove catch-all case in runAction