UI Nits: Sidebar remember their state

Rather than relying on the parent to maintain the splitter sizes
and states, the GcSplitter remembers its own state and sizing.

This includes remembering if the children have been hidden via the
icon bar at the bottom of the splitter.
UI Nits: Stop unhiding the sidebar

.. when switching views. It seemed like a good idea at the
time, but now it is really irritating. If I want the sidebar to
be shown I'll select it thanks.
UI Nits: Images embossed

.. akin to the NSImage 'template' function we now pre-process
the GcSplitter icons to get a shaded and embossed look. Its a bit
clunky code, but its only run at startup.
UI Nits: Center Icon Bar