ImplImageTree was used outside of VCL which is not consistent with
the name and the header also contains a lot of implementation
detail. This separates the implementation to ImplImageTree and
the public interface and singleton to ImageTree only.

Change-Id: I3a26444f0f6971a6b1d83472e9cef19c93192d3e
Reviewed-on: https://gerrit.libreoffice.org/32134Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>