ScreenDiags as Entities rewrite to allow for multiple windows

[laundmo]

now that camera-driven UI landed in 0.13 theres a potential usecase of rendering different diagnostics to different windows, or even just the same diagnostic to multiple windows, which is not possible due to the UI only having a single TargetCamera.

Each diagnostics text collection should be represented by a entity instead of a resource, to allow multiple to exist. this would allow for both the usecases mentioned above.