Add tag add/modification/delete targets to Notifier

[dstillman]

Need way to get tag events independent of item change events for the tag selector, and utility writers will probably want it too.

[dstillman]

(In [1009]) Closes #470, Add tag add/modification/delete targets to Notifier
Closes #471, Tag selector should update when tags are added/removed

Tag Selector overhaul:

• Right-click to rename/delete tags globally
• Filter tags to only those associated with currently visible items, with a Display All checkbox to show others in gray -- scope list set via new callback mechanism in the items tree
• Drag and drop items onto tags to batch assign
• Tag Notifier events, currently unused (tag selector currently just refreshes on all item events, since doing granular tag updates is considerably more complicated)
• Performance improvements, offset by the new features that make it slower

There should probably be an option to use either an ANY or an ALL search in the tag selector... (It's ALL by default now.)

New methods:

• Zotero.hasValues(obj) -- return true if an object (/associative array) has at least one value, false if not
• Zotero.Item.addTagByID()
• Zotero.Item.hasTag()
• Zotero.Tags.getAllWithinSearch(search)
• Zotero.Tags.rename(tagID, tag)
• Zotero.Tags.remove(tagID)
• ItemTreeView.addCallback()
• ItemTreeView.setFilter('search'|'tags', data) -- replaces searchText()
• CollectionTreeView.getSearchObject() -- search object used to generate the items list
• CollectionTreeView.getChildTags()