VirtualKeyboard

Hi Bruno,

this provides a first shot of an in-app virtual keyboard and its use in Gletters for mobile devices.

Some notes, besides the ones in the commit msg:

- A shift key function probably should be added to avoid tooooo big keyboard layouts. If you try level 10 with default-fr.json in Gletters you'll see what I mean. :-) Also, populateKeyboard() takes pretty long for such huge layouts (on my Samsung Galaxy S2). This is the only real issue I still see.

- Design can surely be improved. I am not an artist ;-)

- Also performance might be further improved. I had to fight with QML especially for the z-value of the keys (inside a row and between rows). More QML experience would be needed for that!

- VirtualKey could be refactored into VirtualKeyboard as a anon Component and probably should unless used elsewhere, I think. 

What do you think?

I'd schedule the shift function if you agree. Any other things you'd like to change?

Regards,
  Holger