NumberInputWidget: Remake as an actual TextInputWidget child

Authored by Mooeypoo on Apr 19 2017, 11:44 PM.

Description

NumberInputWidget: Remake as an actual TextInputWidget child

As the name of it suggests, it should be a decendent of InputWidget
and, more specifically, of TextInputWidget. This solves a bunch of
compatibility issues.

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)

Bug: T124856
Bug: T149644
Change-Id: I00164fcaf5092b60243d4e3cdc8992285cee33b4