Would be useful to expose maximum OSK height

There is a request from Design that on the greeter, we position the password input box above the OSK (so that it never has to move when the OSK appears).

For unity8, we could probably hack together something that makes assumptions about size of OSK, but that's brittle, especially considering the future, when we might have input methods or custom keyboards.

So I'm proposing that we add an UbuntuKeyboardInfo.maximumHeight field (and I guess maximumWidth to go with it...). This would need support in ubuntu-keyboard too.

I don't know the OSK design space super well though, so please provide commentary if this could be done a nicer way.