Enable multi-select in Tags tree (rebased onto dev_5_0)



This is the same as gh-2446 but rebased onto dev_5_0.

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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.