- Add new enum GimpComponentType which contains u8, u16, u32 etc.
- Change GimpPrecision to be u8-linear, u8-gamma, u16-linear etc.
- Add all the needed formats to gimp-babl.c
- Bump the XCF version to 5 and make sure version 4 with the old
  GimpPrecision enum values is loaded correctly

This change blows up the precision enums in "New Image" and
Image->Precision so we can test all this stuff. It is undecided what
format will be user-visible options in 2.10.