New "Apply watermark" checkbox

Okay the new feature was a bit tricky to implement, but here is my latest patch.
This patch is for 5.x and HEAD and can be ported to 4.7.

new feature

* manage watermarks: checkbox "Apply watermark" for users with "manage watermarks" permssion. (Known minor bug by design: If you want to replace an image and "Apply watermark" is checked, when upload fails, e.g. because of wrong file type, watermark is applied to the image that you wanted to replace.)

fixed

* watermark applies once on preview, but twice on submit

changes

* hook_nodeapi rework and some structural changes