core: Bar: add repeat button
Ported click_on_letter activity

ToDo / Open Issues:

- Sound with qrc source schemes not working on Linux
(see https://bugreports.qt-project.org/browse/QTBUG-36e724).
- No voices sounds (on linux) unless symlinked manually.
- "fx"/sound settings faked by getSettings(), need settings
integration.
- No scaling on Linux because ApplicationInfo.ratio is never updated.
- Levels are statically initialized. Allow to initialize from file
(for non-latin locales), probably needs file IO support from C++.
- Flicking the train on Android can be improved.
algebra_by, improved text size

The new text size fit better on hdi screens.