qml: Hookup keyboard

So here is some basic keyboard handling.
Still some issues to solve:

1. Changes don't get drawed immediately, you have to scroll away and back again. (Should be part of another pr)
2. I am not sure how to handle the key/modifier combination properly. ATM pressing Shift+Z will
send a `KeyPress{Key: 'z', Shift: true}`, but the editor does not convert that to uppercase `Z`. So
in that case I should use the [KeyEvent.text](http://qt-project.org/doc/qt-5.1/qtquick/qml-qtquick2-keyevent.html#text-prop) property. 
    1. So how to decide whether to use the text only, the key/modifier combination or maybe even a combination of both?
    2. What KeyPress does the editor need to handle e.g.: "ctrl+shift+c"
3. ATM all key presses in the LUT are accepted, that means ALT+X for menu navigation
would not work. What I would need is feedback whether something is bound to that key combination or not. If not
it would pass the event to other widgets, by setting event.accepted = false.

First go code ever here, please help with idiomatic issues.

Looking forward to your comments.