i18n: use locale().decimalPoint in display
Propagate display value to the Application

This is needed to let the cover display the current calculation value.
Display current value on the cover page

Rework the cover page to show the current value if available; add some
texture to the cover page so that it can be recognized as a calculator.
Merge branch 'feature-cover' into develop

Merge finished development of the cover feature.
Create a new KeyButton component

This is needed to add haptic feedback to each keypress
Add new components for build
Use new KeyButton in the calculator
Merge branch 'feature-keyboard-feedback' into develop
Bump version to 0.4.0