Fix_157: Create FlxToolbar. Repeat of #160.

>> Fixes #157, Repeats #160

I would suggest seeing #160 for an excess of discussion about roughly the same work.

Recommendations moving forwards...

Are we merging in Flixel Power Tools? (I suggested this in #163) Most of the logic here is really a fudge that breaks what I feel is proper object orientation; buttons should be responsible for themselves and FlxButton from the Power Tools kit would be - in effect - what I'd end up writing if I'd extract them to do as such within the scope of how Flixel works. I believe @IQAndreas had hinted at similar work (and a few ideas were tossed around besides) in #160's conversation.

Beyond that the new class is semi-abstract. I've sort of cherry picked what I feel is correct for a parent (or, rather, not incorrect) and this results in a couple of functions that exist purely as placeholders (their bodies will have a "pass" comment).

If there's any notes on this work or anything that I could do let me know. I have done preliminary testing and everything seems sound - however, it never hurts to do extra testing, especially for something so important :)
This `return` should not be here. The same code block in `Vis` is correct.