Remove CombinedColorAdjuster

Now replaced with workspace tabs for each individual adjuster.
Misc widgets: convert to GTK3
workspace: use a common properties button

Widgets should have a common way of exposing their properties for editing.
Place a button next to the close button to do this.
toolbar: split out the special factory action

Make it more generic, since it could be used for proxy menu items as well
as toolbar items.
app/tdw [GTK3]: fix cursors not compositing

Different return tuples in PyGI+GTK3.
Add assorted extra debugging.
Statusbar: add extra required widgets (NFC)

Added (to Glade, and with supporting code for the MyPaint runtime):

* Color picker button (icon taken from the Inkscape distro; GPL2+)
* Previous/next preview indicator (which might evolve a bit)
* Add missing Glade icon for the drawing window.
Make footer/Status bar useful

All very fluid right now, things may change. But for now:

* Textual status bar shows the current mode and what you can do in it.
* Bottom left corner has colour picking and preview stuff
  * A picker button (which we may make configurable...)
  * Prev/next color preview button
  * Button for bookmarking the current colour
* Remove footer widgets from all adapted classes, since the common stuff
  is now available in the status/footer bar.
Merge MyPaint master up to 1aa0765

Conflicts:
	gui/drawwindow.py