Add new virtual function GimpDrawable::convert_type()
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.
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