Dataset creation 11048

Mainly addresses http://trac.openmicroscopy.org.uk/ome/ticket/11048
Web now allows you to create a Project, Dataset or Screen regardless of what is selected in the Tree (both via right-click menu or toolbar).

To test: If you have a Project selected and create a Dataset, this becomes a child (no change in behaviour) but if you select Images and create a Dataset, the Images will be added to the Dataset.
In all other cases, empty P/D/S will get created as an empty orphan container.

Other commit is a very minor edit to Blitz Gateway to handle None in annotation.getValue() cc @chris-allan http://trac.openmicroscopy.org.uk/ome/ticket/10666   but this is tricky to test (need to create an XmlAnnotation with value of None).