[Delivers #46462289] jstree multiselect patch

A few tweaks to the tree javascript to support multiselect better.. namely:

- if Mac OS X, use the META key as the "add to selection" key, CTRL for all other OS (CTRL in OS X invokes a menu - best not override this action)
- if CTRL+click used to deselect, ensure tree is updated to reflect this change
- don't allow CTRL+click to deselect the primary selected node - as this node cannot be deselected.
- allow 'Move' toolbar action to work with multiselect.
- if "Move' > 'Down Into...' is clicked where the target is one of the selected nodes, then deselect the target to allow the remaining selection to be moved into the target.