Enable multi-select in Tags tree

See http://trac.openmicroscopy.org/ome/ticket/11668
This enables multi-selection in the tree on the Tags page.

To test, browse tags that have multiple images under them, select multiple images. Right-hand panel displays "Batch Annotate" panel as normal, for annotating images.
NB: If multiple Tags are selected, right panel is blank since it doesn't support batch annotation of Tags.