Add TransferValueOnCommit support to TextBox

[peppy]

Currently, TextBox updates its attached Current bindable immediately on any change. This is rarely what we want. A pattern similar to SliderBar should be employed to allow the consumer to toggle transfer-on-commit.

Probably needs some refactoring of the "instantaneous" bindable featured in SliderBar into its own tested class.

Use case osu! side: editor timing screen needs to know when a final value change happens to track state changes, but can't do so without manually intercepting the commit event currently.