Move thumbnail creation into ShellTextureCache. It's now asynchronous,
and we cache the result.
Create a DocManager class which keeps around the DocInfo objects between
invocations. This is also where we ensure we remove thumbnails for
recent items not known anymore.
Add a ShellTextureCache class which loads (and can cache)
pixmap->texture conversions. This fixes a problem with the
async code in ClutterTexture that it was lower priority
than animations, and also ensures we're really only
loading these pixbufs once in the icon case.