ENH: Various minor features and bugfixes for subject hierarchy 2.0

Details:

BUG: Update subject hierarchy item info label when item is changed

Also added missing observations for items that are created when importing the scene

ENH: Improved usability for registration subject hierarchy plugin

A Cancel action appears when right-clicking the 'from' volume that allows canceling first selection, and also explains what to do to continue with registration

ENH: Changed subject hierarchy scene separator characters to support URLs

When serializing a URL, the character ':', which is the same as one of the separators, would confuse the deserializing function

BUG: Fixed issues in subject hierarchy display visibility

The symptom was that after display visibility was set, the item was not selected, thus there was a flickering of the data node inspector. The underlying cause was that subject hierarchy node modified event was invoked from the set visibility function that updated the whole tree.
Updating the whole tree now does not happen if the node is modified. Instead, the modified event is used to indicate to the scene that the content of the node was changed in some way. Thus, subject hierarchy node modified event is invoked in case of any subject hierarchy item event.
Also the parent items' icon was not updated when the visibility of the children changed. The previously existing mechanism was reinstated in SetDisplayVisibilityForBranch.

BUG: Fixed scripted subject hierarchy plugins on Mac/Linux

ENH: Added subject hierarchy node function to get number of items

ENH: Disable showing root item by default

ENH: Subject hierarchy folder plugin edit properties works

If a folder node corresponds to a model or annotation hierarchy node, Edit properties switches to Models or Annotations, depending on the hierarchy node

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25759 3bd1e089-480b-0410-8dfb-8563597acbee