Allow setting number on choose-elem-button

[Sharparam]

Currently there is no way to display a number/label on a choose-elem-button. This is needed e.g. for making a custom combinator GUI where you can select signals and they also have a count associated with them which needs to be displayed (just like a vanilla constant combinator).

Currently you can work around this issue by making your own label inside the button, but this is a hack and it's far from ideal (the text is sometimes truncated when too long for example, something that doesn't happen with vanilla GUI buttons).

(This is related to viewtopic.php?f=28&t=122689 but for the opposite case basically.)