This commit makes GTK_ALIGN_START/_END pay attention to
the text direction when used in horizontal context.
This is how similar parameters in GtkMisc and GtkAlignment work,
and is generally expected of GTK+ positioning parameters. And this
is new GTK+ 3 api, so it is basically still unused at this point.
If explicit right/left turn out to be needed at some point, we
can expand the enumeration with new values.
There are two colored boxes with toggle buttons nested
inside several GtkBox. Toggling these to expand mode
should automatically propagate expansion up through
the several GtkBox such that resizing the window
results in resizing the colored boxes.
https://bugzilla.gnome.org/show_bug.cgi?id=628902
