Emit notify::label in GtkMenuItem also when label is changed through GtkAction.

Refactor GtkMenuItem and remove duplicated code for GtkLabel creation. Reset
the accel-widget back to the GtkMenuItem itself when there is no action related
to the GtkMenuItem anymore.

Add test for notify::label emmisions.

Fixes bug 612574 - GtkMenuItem does not emit notify::label when label is
changed through GtkAction.