Update button.ts
Update button.ts

Added static "CreateImageWithCenterTextButton" with centered text and full-size image background.
Which i think is a very common button setup in any game :)

I used zIndex to render text infront of image instead of adding it as second control.
this way we keep consistency with other text buttons, and can change .text value in the same way & child id, e.g. button.children[0].text

https://www.babylonjs-playground.com/#XCPP9Y#288
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