Avoid NPE when opening a tree editor under Linux

This is a temporary workaround for a regression in Kepler, just to see
how much it improves the situation (or not). See also the discussion at
http://www.eclipse.org/forums/index.php/t/560710/ and
https://bugs.eclipse.org/bugs/show_bug.cgi?id=378846#c16.

Change-Id: I66e64db9fe02dde69684b7a1d86efd67941794e1
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
Do not load resources already loaded in ModelUtils

This can cause problems when Xtext is present and imposes its own
XtextResourceSet, which throws an exception when it sees duplicate
resource URIs.

Change-Id: Id5de1acb754c3313affeed266007853127777465
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>