Each menu is a focus manager group, but there is also some explicit
focus handling between non-hierarchically-related widgets. Eg, to move
between menus, or from a menubutton into its menu.
https://bugzilla.gnome.org/show_bug.cgi?id=621671
Icons can be loaded as St.Icon.SYMBOLIC, FULLCOLOR, APPLICATION or
DOCUMENT. The first will look for a symbolic equivalent, the second
looks for a full-color version (and does fallback, eg, from
"drive-harddisk-usb" to "drive-harddisk"). APPLICATION and DOCUMENT do
full-color icons without fallback (as specified by the icon spec).
And update various callers to use the right flags.
Based on a patch from Matt Novenstern.
https://bugzilla.gnome.org/show_bug.cgi?id=621311
Solved by splitting the base class (PanelMenuButton) in a separate
module, ui.panelMenu, which is meant to hold also other reusable
button classes.
https://bugzilla.gnome.org/show_bug.cgi?id=621705
