[Mobile UI] Introduce separate keypad fill modes (#297)

[Vogtinator]

Allow ResizeToFit in addition to the current mode.

Draft because:

• [ ] Investigate better implementations, this adds another keypad instance
• [ ] Landscape mode not taken into account
• [ ] The keypad widget in the desktop UI could maybe benefit from this as well

[HimbeersaftLP]

Would it also be possible to scale down the screen instead of the keyboard?