COMMON: Add SeekWhence enum to replace SEEK_x macros

The whence parameter in seek calls has been replaced with
Common::SeekWhence. Most client code has been updated to use
this enum instead of the SEEK_x defines.
GOB: Use SeekWhence enum instead of SEEK_x
SCI: Use SeekWhence enum instead of SEEK_x
WINTERMUTE: Use SeekWhence enum instead of SEEK_x
JANITORIAL: Remove SeekWhence FIXME comment
JANITORIAL: Remove explicit Common:: in common files