Fix spelling and grammar in various places.
Fix spelling and grammar in various places.

I tried to make sure things were consistent (e.g., same parts of
speech within a group), sentences made sense, etc.

achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint