Support for BMFont in ui::TextField

When searching these forums and spitting through the code, I found out that it is not possible to use a bitmap font for the ui::TextField. It shouldn’t be too hard to support it, as internally it uses a Label which already supports the BMFont. I was wondering if other people would also appreciate support for this. If so, perhaps I can create a PR for it. If not, I will hack my own version based on the current implementation. Or perhaps I missed something and there is already someone working on this.

Ok, so it seems there is not a lot of interest for bmfont support. I will still create a PR for it, it seems like a missed chance to not support it (there are only a few minor changes needed).

I created a PR for this functionality, see https://github.com/cocos2d/cocos2d-x/pull/18587