BUG: Subject hierarchy UI update issues fixed
Fixes #3714

- qMRMLSceneSubjectHierarchyModel::nodeIndex rewritten to generate correct indices, parentNode also fixed. Workarounds removed
- New subject hierarchy nodes get simpler names
- Setting icons is not so simple as it should be due to a bug in Qt (https://bugreports.qt-project.org/browse/QTBUG-20248), which raises the need to check if the icon to be set is the same as the current icon of an item. This introduces one more thing to remember to the developer. Thus, instead of having the plugins set icons on items, they just return it (or an empty one), and the model does the check. This way developing plugins is not harder than before.
- Changed copyright information in all subject hierarchy files
- Instead of having the potential nodes list on the bottom of the module, those nodes are shown in the subject hierarchy tree on the top-level without icons. They can be shown/hidden with a checkbox under the tree, and can be handled the same way as in the potential list before.


git-svn-id: http://svn.slicer.org/Slicer4/trunk@23489 3bd1e089-480b-0410-8dfb-8563597acbee