Add ktx-artemis module

[czyzby]

• Adds ktx-artemis to third-party libraries.
• Fixes Box2D dependency in TeaVM backend #106.
• Uses Gretty version from advanced settings in GWT to ease maintenance.
• Updates Kotlinx Coroutines.

[czyzby]

I've added it very recently along with the TeaVM backend. It's not in the UI though, I just added it as a property to ease maintenance.