Simplify toDataElement

Replace 'type': 'foo' with 'type': this.name where applicable. This
makes inheritance easier.

Also implement { 'type': this.name } as the default in ve.dm.Model and
remove any implemenetations that don't override this.

Change-Id: I54943588c5cc23218ae1332a137b311fb4608b5f
Merge "Simplify toDataElement"