Make ValueChanged give an EventArgs

[smoogipoo]

ValueChangedEvent<T>:
    <T> OldValue
    <T> NewValue

And also forwards compatible if we ever add anything else to these args (or override them or something).

Interested to hear everyone's thoughts on naming/etc.

[UselessToucan]

You wrote earlier that we don't use EventHandler/EventArgs.

[smoogipoo]

@UselessToucan Of course, I can remove it, but EventArgs is a dummy object by itself

[peppy]

Let's go with ValueChangedEvent to reduce the length of the class and follow what we are using elsewhere (UIEvent for instance).