Add close button/action (Ctrl+W) to model editor tab

[adamkewley]

Estimate: 0.25d

The model editor tab does not have a close item in the file menu at the top. This is inconsistent with the mesh importer, which does.

This will require adding it to MainMenu.cpp:MainMenuFileTab. Example implementation can be found in mesh importer (MeshImporterTab.cpp), which has a Close implementation that uses RequestClose.

This might require injecting the TabHost somewhere, because it will require having the close button request that the model editor tab is closed.