accel label: Fix RTL handling

accel label: Fix RTL handling

The 'mad hack' that GtkAccelLabel used to affect the GtkLabel
draw function broke with the introduction of gadgets, since
the positioning is no longer relative to the widgets' allocation
at the time of the call, but rather to the gadgets allocation.

Instead of coming up with an even madder hack to keep this
working, give the GtkLabel draw function knowledge about accel
labels.

https://bugzilla.gnome.org/show_bug.cgi?id=760663