windowing: remove uses of WindowWithSavedPosition

Remove uses of WindowWithSavedPosition outside layout.py - which will be
removed shortly.
workspace: integrate tab-based layout replacement

Begin using the new gui.workspace layout code. This replaces a large chunk
of old, crufty code with a nicer system that makes better use of the screen
space and is built more closely on top of internal Gtk types.

User improvements/changes:
 * Sidebars are now present on both sides of the screen.
 * Drag tabs around to reorder and to move docked tools between
   sidebars and a floating window state.
 * Fullscreen is a bit more understandable: there's an auto-hide timeout,
   and poking the sides of the screen or hovering over a hidden floating
   window reveals the hidden controls.
 * Fullscreen state is remembered between invocations now.
Merge branch 'mypaint-head'

Port animation window to the (much nicer) new MyPaint window system

Conflicts:
	gui/application.py
	gui/drawwindow.py
	gui/resources.xml
	gui/toolbar-subwindows.xml