Add ability to take a screenshot about idb HOT 3 CLOSED

Screenshots should certainly be possible one way or another:

1. Been thinking about interactions generalised to return 'id'. This could make more sense for some people rather than shoving the interesting information in the session state.
2. if it's not possible to use DVTDevice, we could screen capture the simulator app (yuck)
3. It should be possible to run a daemon process on the simulator that has access to the framebuffer (IIRC WebDriverAgent does this)

cc @mmmulani

from idb.

This should be complete when I:

1. Fix the callsite FBWritableLog
2. Add On-Demand fetching to FBSimulatorFramebuffer so that the latest image will always be used in preference to the cached value.

from idb.

This exists in FBSimulatorDiagnostics

from idb.