UI improvements to New <Resource> wizards: titles, no duplicates

Make some improvements to the New Source Folder/PyDev Package/PyDev Module wizards:
1. add persistent titles to the wizards.
2. no longer allow creation of duplicate resources. (Before this patch, giving a new
resource the same name as an existing one was valid, allowing paths to be written multiple
times to the PYTHONPATH.)
3. prohibit selection of multiple items in the PythonPackageSelectionDialog.
Merge pull request #41 from AndrewFerr/aferrazz_dev

UI improvements to New <Resource> wizards: titles, no duplicates