Handle on_tap in RowType::Base

Preliminary so you can have a look.

Not a bug per-se, but as of now only RowType::ButtonRow will respond to on_tap. In my case I want to have a StringRow (or StaticRow rather) switch to a new view on tap rather than edit in place. 

Maybe this is only good for StaticRow but I wasn't sure. If on_tap is not defined, no effect. Shouldn't affect any custom rows that wouldn't call super anyway since there wasn't logic before in Base.