Add the ability to have two numbers on sprite-buttons

[Therenas]

At the moment, a mod can add a number to the lower right corner of a sprite-button by using the 'number'-attribute. The new character UI of the base game has sprite buttons with two different numbers, stacked on top of each other. I would like to have that possibility too for modded sprite-buttons. I have several concrete use cases already in mind, but the utility of this should be pretty apparent.

[Rseding91]

Currently literally only that exact 1 button in the character GUI supports drawing multiple numbers in that style. To make every other place support it just for modding support isn't likely to happen.