(bug 45159) Separated toolbar for editing a reference's snaks

Instead of having a simulated main snak, a toolbar above a reference's list of snaks
will control starting/stopping edit mode for the reference's snaks.

Change-Id: I3b31dd7e240a779b2a036e43cc568d0ed3709c02
Merge "(bug 45159) Separated toolbar for editing a reference's snaks"