Commit 08690d658f42 generalized the banner-blocking behavior of the
dateMenu to all menus that would obscure the banner. However setting
up the 'open-state-changed' handler only when an indicator is added
does not work for indicators that change their entire menu (like the
app menu) - we currently end up with menus with no connected signal
handler, and throw an error when trying to disconnect an invalid
signal ID.
To address this, add a new PanelButton::menu-set signal and use that
to set up the 'open-state-changed' handler.
https://bugzilla.gnome.org/show_bug.cgi?id=745910
1092f55b54