Similar to the latest commits for GimpBrush:

- gimp_pattern_get_buffer() returns a GeglBuffer and allow getting a scaled
  version of the pattern.
- Old gimp_pattern_get_pixels() is made private.
- Moved GimpPattern into its own file and store the buffer to avoid re-querying
  it through PDB continuously.

No as for the widget to select a pattern:

- Preview frame ensured to be square.
- Default size increased.
- Drawing code using the new gimp_pattern_get_buffer().
- Cleaned up code.