files-view: Add gestures to open background menu in empty view

Commit c653f880 ("list-base: Don't inherit from NautilusFilesView") broke our ability to right click on an empty view to open the background context menu because the list-base is no longer a child of files-view. We could move the gestures from list-base to files-view, but that's problematic not only because of the weird hack we use (to deny background click after clicking on a view item, but also because we might want to not allow background click (for example in the network view), and claiming the background click in such a view would interrupt the gesture for the underlying view (i.e. GtkListView).

So let's take the path of least resistance and just add the gesture on the empty state itself.

Fixes: #3281 (closed)