Introduce gtk_tree_view_is_blank_at_pos()

Introduce gtk_tree_view_is_blank_at_pos()

This function is useful to figure out whether the tree view is "blank"
at a given location.  For such locations you might want to popup a
custom popup menu, clear the current selection or start rubber banding.
In the future, we are planning on updating GtkTreeView's user
interactions to take advantage of this new function.

Part of bug 350618.