auto-spell: use a GtkTextRegion for the region to scan

The most basic implementation with a GtkTextRegion. All the region is
scanned directly. The plan is to:
- Scan the region after a timeout in some cases (insert-text,
  delete-range, etc);
- Scan only the visible regions on the screen, instead of scanning the
  whole buffer. Scanning one screen of text should be fast enough to not
  block the UI.