[duim,gtk] Migrate do-copy-area

When the from-medium and to-medium coincide follow the example in the migration
guide: https://developer.gnome.org/gtk3/3.5/gtk-migrating-2-to-3.html
Merge pull request #543 from fracek/duim-gtk-fixes-4

[duim,gtk] Migrate do-copy-area
Begin making duim-reference use Sphinx.
[gtk] Remove things that aren't present on OS X.

2 of these will return when they're implemented upstream.
[gtk] We don't need anything from xlib.

We also don't need it at link time apparently (tested on Linux).
On Mac OS X, we are using the Quartz backend, so we don't want
xlib there. Nor would we want it on Windows.
[gtk] glib depends on gobject for some symbols.

This addresses a link error on OS X.
[gtk] A temporary hack while we still use gdk_threads_*

This will go away in the future. It is a workaround for a link
error on Mac OS X where we can't have undefined symbols in
our current build system arrangement.
[duim,gtk] Add duim registry entry for Mac OS X.
[duim,gtk] Import Gtk+ with subtypes superclasses

Objects interfaces were missing.
[duim,gtk] Fix compiler crashes when compiling gtk
Merge pull request #553 from fracek/subtype-super-classes

Subtype super classes
[duim,gtk] Fix error clicking in tree model.

Closes #554.
[duim,gtk] Fix export of renamed handler.
[duim,gtk] No need for open adjective here.

We don't extend these outside of the library.
[duim,gtk] gtk_widget_size_request() is deprecated.

We use the suggested replacement, gtk_widget_get_preferred_size, instead.
[gtk] Stop exporting bindings that don't exist now.

We removed these due to issues on Mac OS X but were still exporting
them.
[doc,hacker guide] Add short section on DUIM / Gtk.
[duim] Remove duplicated word in outline.
[duim,tests] Turn off threads in the test harness.
[doc,hackers] Make note of the 64 bit issues for GTK+ on OS X.