NumberInputWidget: Remake as an actual TextInputWidget child
As the name of it suggests, it should be a descendant of InputWidget
and, more specifically, of TextInputWidget. This solves a bunch of
To demonstrate this, a demo was added with TagMultiselectWidget
using NumberInputWidget; in its previous version the NumberInputWidget
could not have been used inside the TagMultiselectWidget because it didn't
have the expected API of TextInputWidget.
Bonus: Renamed the horribly named 'setIsInteger' to 'setAllowInteger'
(and the getter as well) and added backward compatibility (mostly
at James_F's behest)