Show loading screen during VFS initialization

Engine does not render anything when scanning data folders and archives. For setups with many folders or heavy archives this may take several seconds. This becomes especially noticeable for Oblivion, Skyrim, Fallout. Also the freeze detector is triggered sometimes by that on Windows. In general this gives the impression like nothing happens or the engine is broken.

Similarly to showing progress of loading content files, the engine should show progress of initializing VFS. Basically the problem is scoped to VFS::registerArchives function call here.