Moved thread control functionality into a separate folder & added threading mode environment variable
Use DALI_THREADING_MODE to control which threading mode to use:
- 0: SeparateUpdateRender: Event, VSync, Update & Render ALL on separate threads.
- 1: SingleThreaded: ALL functionality on the SAME thread.
- 2: CombinedUpdateRender: Three threads: Event, VSync & Combined Update/Render
Only Mode 0 supported in this patch, modes 1 & 2 activated in later patches.
Change-Id: Ib45e7f5305ae56e9bd79def81a61503998e34824