Bug 735906 - Transform tools give unexpected results when transforming...

...certain sets of linked layers

Fix the move tool (GimpEditSelectionTool) using the same principle
as the other "linked item" features, just a bit more complicated...

Never translate the active item and its linked items separately,
always translate the entire list at once.

The linked logic was distributed across the entire file. Changed the
code to prepare lists of items that are translated live (layers and
vectors), and items that are translated at the end (channels, masks
and the selection). In the motion and button release functions, simply
use the prepared lists without any further duplicated checking.

Also clean up the stuff a bit, there is more cleanup needed but first
the fix...

(cherry picked from commit 358f13f5)