Add a new GimpAccelLabel widget, which shows an accelerator label
for a given GimpAction. Unlike GtkAccelLabel, GimpAccelLabel
doesn't show a user-provided label in addition to that.
Note that the size request of GtkAccelLabel doesn't include the
accelerator part, which is desirable in some contexts.
GimpAccelLabel doesn't suffer from that.
(cherry picked from commit 2259ad5fcc)
