Fix bug in jquery plugin for saving page sort order.

Apparently, the new page sorting algorithm was orinally made for Alchemy 2.6,
which used ```_recursiveItems(li)``` - in 3.0, the signature of that function had changed
to ```_recursiveItems(item)```. However, some of the code *inside* the function
did not appreciate that changed and barfed. I did not find this bug on my own, but had help
from  @acetoxy and @thereisnospoonpt (who wrote the original sorting algorithm patch). Thank you!
Bump version to 3.0.0.rc8
Refactors the dialog, so it is scrollable if the dialog content is larger than screen height.
Make all dialogs draggable.
Prevent scrolling of body, if dialog is open.
Updates Dutch translation with more translations, fixed some typo's