Add a Meshes panel to the model editor.

This is work towards resolving issue #50. The idea is that the use will select a mesh and then set material properties on it.
Fix a bad bug where shortcuts used in a scene editor would be handled by all other opened scenes also.
Redesign the way materials are set on meshes in the model editor.

This is work towards resolving issue #50.
Add a minimum height to the bones tree view in the model editor.
Add the ability to see the uniforms of a meshes material in the model editor.

The values can not yet be modified. This will be implemented in the next commits.

This is work towards issue #50.
Finish adding support for setting custom uniform values for mesh materials in the model editor.

This changes the format of .gtmodel files. They will need to be deleted so they can be re-created with the new format.

This resolves issue #50.