Replace FrameContext with Context

[davesmith00000]

Goals:

1. Introduce the idea of services, which should be stub-able to replace the parts of FrameContext that were stateful or side effecting.
2. Rework FrameContext into Context, something better organised / designed.
3. Also split random number generation into Dice (pseudo random and imperfect, but stateless) and Random (stateful).

Related issues: #783, #742, and to some extent #750

[davesmith00000]

Brief summary:

FrameContext replaced with Context. Context is split into frame and services, which are frame values and long running processes respectively, accessible something like this:

// `StartUpData` is accessible at the top level
ctx.startUpData

// Example 'frame' values / facilities
ctx.frame.time.delta
ctx.frame.dice.rollDouble // PRNG seeded on the game running time at the point of frame entry

// Example 'services'
ctx.services.screen.capture(..)
ctx.services.random.nextInt // Long running scala.util.Random instance.

Also, everything has an interface, meaning that it is much easier to include in unit tests.