Added variable names for window items.

Has not been tested will require modificaiton of any references to these items.
Added viewport_target_sprite variable name.
Merge branch 'window_h' of https://github.com/duncanspumpkin/OpenRCT2 into window_h
Removed compile errors. Implemented changes to window.h
Merge pull request #229 from duncanspumpkin/window_h

Window h and compile error cleanup