...certain sets of linked layers

Fix this bug for flip, rotate and general transforms (not for move yet):

gimp_item_linked_flip,rotate,transform(): always transform the passed
item too (do not filter it out of the list of items), so these functions
do the entire job of transforming a linked group now. Transforming the
active item separately didn't work (and is not implementable) if both
a layer and its parent layer group were linked.

flip tool, transform tool, layer->transform callbacks: don't call
gimp_item_foo() *and* (if the item is linked) gimp_item_linked_foo().
Instead call gimp_item_linked_foo() if the item is linked, and
gimp_item_foo() otherwise.

This commit also kills the mis-feature of transforming the selected
pixels of the active layer, and then the linked items completely. We
now either only transform the selected area *or* the linked group.