Add DocumentApi::bgColor() member functions
Remove invalid comments in a couple of files
Remove bgcolor param from DocumentApi::backgroundFromLayer()
Simplify DocumentApi::clearMask(): we just need the "cel" param
Fix compilation error in DocumentApi::clearMask()
Fix problems clearing background cels

- The background color to clear is calculated in one place: DocumentApi
- Rename RemoveCel command to ClearCel
Fix DocumentApi::copyFrame() and addEmptyFrame()
Fix some cases of copy_range() of frames
Save color palette in RGB .ase files (fix #487)