According to the GTK docs, the handler is supposed to have a gboolean
return type and return FALSE to propagate the event further.  With the
previous function signature, on some setups (such as Fedora 31 and 32)
the about dialog would show up as a black rectangle or be invisible.

Change-Id: I3d0d343df58b08f8cc2e7a9f97710dacc63f1c41