Do not try to display the edit bar for synthentic resources

Fixes #16
Do not display a property listing if we can't get hold of a
propertyIterator.

Relates to #16
Fix #22: Node creation: available node types should be picked up from
the repository
Merge pull request #23 from rombert/issues/22

Fix #22: Node creation: available node types should be picked up from the repository
Merge pull request #19 from rombert/issues/16

Gracefully handle synthetic resources