Persistent scene for achievements system

[DivineOmega]

Achievements are so common in modern gaming, that building a persistent scene for Covergame to handle achievements would likely reduce a lot of re-inventing of the wheel. This should include the following functionality.

• Adding of achievements
• Triggering of achievements (with optional rendering of an achievement status bubble)
• Display of an achievement browsing user interface

We could also integrate into:

• Steam achievements via https://github.com/greenheartgames/greenworks