Consider detaching game from DevToolsService when not needed

[filiph]

What could be improved

Not a big deal, but currently, when running a game in debug mode, any FlameGame that is ever started will forever be retained in memory unless replaced with a different FlameGame.

From FlameGame's constructor:

    if (kDebugMode) {
      DevToolsService.initWithGame(this);
    }

There is no DevToolsService.detachGame() or anything similar.

Why should this be improved

This can be confusing when one's looking at memory (albeit I agree that one should be normally doing that in profile mode) and sees a game that's supposed to be gone a long time ago, and won't go away even after manual GC.

This almost sent me on a wild goose chase for a giant game-sized memory leak.

Risks

• Some people might want to inspect their game after it's been disposed?
• Could introduce a DevTools connector bug where game's disposed too soon, for example.

More information

I'm not sure myself if this is something worth addressing. Just putting this here before I forget.

Other

• Are you interested in working on a PR for this?

[spydon]

Sounds like a good idea, shouldn't be too hard to fix up!
I'll assign you to the issue.