gtk3: Provide a background-color for menubars

https://bugzilla.gnome.org/show_bug.cgi?id=730016
new spinner design

https://bugzilla.gnome.org/show_bug.cgi?id=730191
Trivial typo fix
set spinner size to 32x32

- it still gets rendered into a pixmap before scaling,
  and the 32 maxsize has been sadly removed in gtk, but
  larger spinner are retarded.
build: fix make dist