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.

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These are just some UI changes I thought were appropriate.