libgimp: add gimp_layer,chanel,vectors,etc,_get_by_id()
which call gimp_item_get_by_id() and additionally check if the returned item has the right type, and return NULL if not. This is both shorter and more readable than layer = GIMP_LAYER (gimp_item_get_by_id (id)); and additionally makes sure we don't cast e.g. a non-layer with GIMP_LAYER(), which will give criticals but shouldn't, because the wrong IDs can come from anywhere and are an input problem and not a programming error (criticals are for programming errors).
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