Disable tab detaching to avoid crash

As reported by Antonio Fernandes in: https://bugzilla.gnome.org/show_bug.cgi?id=781991#c1

Detaching a notebook tab into a new window through DnD leaves Nautilus in an inconsistent state leading to a crash after loading a new location.

As opening a folder into a new window should be done through its action already present in the folder context menu of both files view and path bar, let's disable this alternative way to do it through tab detaching, as Nautilus code was not expecting that use pattern.

Closes #137 (closed)