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.
alignment/margin vfuncs adjust_size_request/allocation
Now get_height_for_width() will internally update the for_width
before passing it to the real height_for_width() vfunc, allowing
margins and extra space for alignments to be stripped, thus requesting
sufficient height for greater than natural widths (and also accounting
for margins properly). Test case adjusted in testadjustsize to ensure
proper behavior.
When fitting a widget into its allocation, the second dimension
is always dependent on the first, so gtk_widget_get_preferred_size()
cannot be used directly (because we want the natural height for
the allocated width, not the natural height for the natural width,
which is generally a smaller height than the height-for-minimum-width
or height-for-allocated-width).
Added test to testadjustsize to ensure proper behaviour.
