Update tree with skeleton
Use a GtkHeaderBar by default
Ensure we return a GtkWidget
Add initial demonstration - very very basic
We need to show a close button by default
Add SidebarWindow skeleton
sidebar: Ensure we update the title automatically
sidebar: Implement most of the logic required for SidebarWindow

We provide a sidebar area, which is actually a GtkRevealer. This may
be used in future to enable automatic hiding/showing of the sidebar
title area.
Note that the title is not yet configurable for the sidebar area.
Introduce gat gresource
Globally load our own CSS

By loading our own CSS within GnomeAppWindow, we can ensure all derived
implementations, like SidebarWindow, are automatically themed/adjusted
as required, ensuring nobody has to continue repeating the same CSS
magic elsewhere.
sidebar: Ensure left header is themed correctly (CSD)
sidebar: Enable changing of sidebar title via GObject property
sidebar: Add accessors for sidebar title

Users of SidebarWindow can now use the _set_title and _get_title methods
to modify the left sidebar title. They should use GtkWindow's set_title
and get_title methods to modify the primary header title.

This commit also fixes a pointer failure where we did not duplicate
the given GValue
demo: Make use of new sidebar_window_set_title function
sidebar: Content should expand and fill
demo: Add a big shiny image. Demonstrate content area
demo: Use a GtkScrolledWindow. Document its fugly shadow.
demo: Fix comment header
sidebar: Remove old default+TODO
Import gnome's autogen.sh