Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.
This is essentially a combo widget containing a single-line GTextEditor
and two buttons for increment and decrement. The GTextEditor::on_change
callback is hooked to prevent non-numeric input but it's not entirely
perfect since that callback is asynchronous. This will work until we have
some more sophisticated input validation mechanism though.
