Expand/collapse multiple selections

[VBelozyorov]

PR for issue #384

Have to little bit change one method of Notifier, I've found two usages of it and seems nothing broke :)

[stennie]

@VBelozyorov The general functionality of expanding/collapsing documents works, but there are two issues to look into:

1. There seems to be an obvious bug where Notifier::initMultiSelectionMenu() is replacing the default context menu of Notifier::initMenu(). For example, try selecting an ObjectId(); the context menu should include _copyValueAction and _copyTimestampAction.

2. To set expectations properly, I think the context menu items should be "Expand Document Recursively" and "Collapse Document Recursively". My initial (incorrect) expectation was the recursive expand/collapse might also expand or collapse the selected subtree of a document.

Thanks!

[VBelozyorov]

@stennie One small issue is still remains: if you select several fields inside one document, then expand / collapse will work only for the field which has been selected first.
The reason of such behaviour in implementation of BsonTreeView::selectedIndex().
For example, same goes if you select _id and some another field in the document: in context menu will be copy timestamp ... item which is specific for fields with ObjectId type.

[stennie]

@VBelozyorov Think can live with that small issue, but feel free to submit a separate PR for it.

Thanks for fixing up support for subdocuments .. very handy!