Input Performance and Stability Improvements

This issue contains all of the tasks for the current User Device Input Path Milestone. This will be updated with any new information as it arises.

[X] Clean up and flatten Input enum/file structure: Try to clean up and coalesce files used by the DeviceInputSystem/DeviceSourceManager
[x] Move Browser-Specific Logic into DeviceInputSystem for more uniform cross-browser experience: Move any browser-specific checks in the code to inside of the DeviceInputSystem to produce cleaner, more uniform code outside of it.
[x] Implement Lazy Picking (Turn on/instantiate picking elements when picking is first explicitly used): Add logic to not perform any picking until a user explicitly references an object for which picking is required
[ ] Improve Input Perf (frame call reduction): Reduce number of times certain inputs are processed to coincide with frame rate

BabylonJS / Babylon.js