add a custom event loop for popups that closes the menu on mouse button
release events and - most importantly - propagates this event back to
any parent menu.
fixes#633
OK - so I have to admit I *hate* the fact that PIP seems to require a
subdirectory named like the library.
But since the PIP package is something really nifty to have (thanks to
@tony again!!!), I updated the codebase to hopefully conform with what
PIP expects. Testruns so far look promising...
