This is a rather big change. Rather than make spawned processes become
children of init, we handle them in xmonad. As a side effect of this change,
we never need to use waitForProcess in any contrib module -- in fact, doing so
will raise an exception. The main benefit to handling SIGCHLD is that xmonad
can now be started with 'exec', and will correctly clean up after inherited
child processes.
If the user used the mouse to change window focus (moving into or clicking on a
window), this should be handled differently than focus changes due to keyboard
commands. Specifically, it's inappropriate to discard window enter/leave events
while the mouse is moving. This fixes the bug where a fast mouse motion across
multiple windows resulted in the wrong window keeping focus.
It's also helpful information for contrib modules such as UpdatePointer - it's
supposed to move the mouse pointer only in response to keyboard actions, not if
the user was moving the mouse.
