use common code for popping-up menus

fix some bugs when the popping-up wasn't made by a mouse click or key
also try to avoid popping a menu too far from the mouse click
mass-tagging dialog : make the '+' button work with the keyboard
gstreamer: make it listen to changes in pulseaudio's volume (only with perl-glib>=1.251)
set role and icon hints for pulseaudio