Visual fixes and multi-touch virtual keyboard

Two small visual fixes:

* Make dimming background of menu black instead of white
* Hide keyboard when window doesn't have focus (looks nicer in switcher)

Also, the virtual keyboard now has multi-touch support:

* Modifier keys can be pressed together with normal keys
* Move touch state handling to Key.qml

Care has been taken to make sure it also works fine in the "old" sticky mode (press modifier, then another key). This behavior should (apart from moving the finger from the modifier to the key to be modified) roughly be in line with what Maliit does.