Removed "Unloading Cell..." text from loading screen

OpenMW unloads the cell so fast, it's hardly noticable. This commit gets rid of
the "flicker" every time a cell loads.
Removing the unloading cells part instead of just commenting them.
Using "Loading Exterior" and "Loading Interior" instead of "Loading Cell"

This commit replaces the default "Loading Cell" text with "Loading
Interior" and "Loading Exterior"
Loading text uses now the corresponding GMSTs
Merge remote-tracking branch 'vorenon/master'
Centered the loading text
Removed the 3 dots after the load text.
Merge remote-tracking branch 'vorenon/master'
Removing all traces of "Open Morrowind".

pvdk said it was ok to remove the header entirely
Merge remote-tracking branch 'vorenon/licensefix'
fix bug 574
Merge remote-tracking branch 'mikesc/bug574'
Don't try to render a map in empty cells