Use gtkcompat.h to slim down a few ifdefs.

Use gtkcompat.h to slim down a few ifdefs.

Now that I've got a general place to centralise handling of at least
the simple differences between GTK 1 and 2, I should use it wherever
possible. So this commit removes just a small number of ifdefs which
are either obsoleted by definitions already in gtkcompat.h (like
set_size_request vs set_usize), or can easily be replaced by adding
another (e.g. gtk_color_selection_set_has_opacity_control).