Price calculator for Upgrade balance

The "Result" label shows the result of R+=P+(LхK), where:

R - result, L - current lvl, K - price koef as price bonus

The K could be different and customised buy lvl label (25, 50, 100 etc)

Tried to make it mobile friendly, so here is the eternal virtual keyboard, that is not mine (https://github.com/martinfuchs/Godot-Onscreen-Keyboard), so could be bit buggy