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
should fix freezes when starting playing a song other than from its beginning

bug introduced by commit #ffffd405e1
(Closes #54)
add compact option to Vpack/Hpack function
make the song properties dialog scrollable
make the songs properties dialog scrollable and make per-song part collapsible
move About button to the other side of the preferences dialog
song properties : move advanced edition button to the action part of the dialog
SimpleSearch: fix suggestion menu flickering with each update
SimpleSearch: small title search optimization in suggestion menu
improve filter cache
add support for filtering in the background
improve responsiveness of SimpleSearch
Merge remote-tracking branch 'upstream/master'
Bump changelog
Merge branch 'master' of github.com:shimmerproject/gmusicbrowser