Bug 544315 - DocumentProviderRegistry: also try IPath to get extension

Make getDocumentProvider(IEditorInput) try a little harder by trying
to figure out an IPath from the editor input and use the extension
of that path, if one can be determined.

Change-Id: I6426e0e9fa09b71bd472d5ca545d1333c17323ce
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
7 files changed