Click on letter

Hi,

this is the first version of click_on_letter.

Pls. cf. git commit msg for a list of open issues/todos. Some of these points could already be integrated before merging upstream:

- No scaling on Linux because ApplicationInfo.ratio is never updated. I think this must be solved on the C++ layer by handling resize events and updating ApplicationInfo accordingly (in QML's Window there seems to be no way to handle resize events). Could look into this ...
- Levels are statically initialized. Allow to initialize from file (for non-latin locales), probably needs file IO support from C++. A simple C++ class for handling file IO could do the job. Could look into this ...

Pls let me know if you want any further changes. After we are d'accord I would also derive click_on_letter-up.

Thanks, regards,
  Holger