Make button height customizable

When you customize the button background image with something too small, the image is resized in a strange way. One possible solution would be to make the resizing cap insets customizable, another possibility is to make the button height customizable, which I'm doing here. Instead of using 31pt as the height, if a "buttonHeight" key is specified in the JSON design file, we use this one, otherwise we fall back to 31pt.