This commit adds a "removed" signal to screens. Together with the "added" signal
that we have since a while, this allows the C code to update the list of
available screens dynamically without needing to restart.
So far, this code received only minimal testing. So far, I don't have a nice
idea on how to easily test this...
Closes: https://github.com/awesomeWM/awesome/issues/672
Signed-off-by: Uli Schlachter <psychon@znc.in>