Add GimpViewable::preview_{freeze,thaw}() virtual functions, which
get called when the viewable's preview is frozen/thawed.  Implement
the functions in GimpDrawable, recursively freezing the parent
drawable's preview (or the image's preview, for top-level
drawables) while the drawable's preview is frozen.  For layer
masks, freeze the associated layer's parent.

This avoids updating layer-group/image previews while painting on,
or applying a filter to, a descendant layer.  This both reduces
lag, and fixes a discrepancy between the layer's preview, which
isn't updated, and its parents' previews.