This may look like duplication of GimpItem::convert() but in fact will
fix the longstanding uglyness that GimpItem::convert() both transfers
an item to another image *and* converts the image type of drawables.
When this refactoring is done, GimpItem::convert() will only move an
item to another image, and its implementation in GimpDrawable classes
will call GimpDrawable::convert_type() to convert the pixels to
whatever format.

Takes a "dest_image" parameter anyway because for converting to
indexed we need the destination colormap. The default impl in
GimpDrawable can only convert to gray and rgb however.