(bug 45159) Splitting snaklistview from referenceview

This change set separates the responsibilities of the snaklistview widget from the ones
of the referenceviewwidget. Now, the snaklistview is actually manages just a list of snaks
while the referenceview just adds some additional functionality regarding the handling of
references. Furthermore, the statementview manages removing the referenceviews.

Change-Id: Id677e6a6387661f00945d6b5dcaa8603705530cb
Merge "(bug 45159) Splitting snaklistview from referenceview"