Added gravity and step listener to physics scene. #224

Job	Run time
Android (Debug)	7m 22s
Mac (Static, -DDEEPSEA_SHARED=OFF)	8m 22s
Android (Release)	7m 39s
Windows (Win32, win32, Static, -DDEEPSEA_SHARED=OFF)	7m 57s
Mac (Shared, -DDEEPSEA_SHARED=ON)	6m 58s
Linux (Static, -DDEEPSEA_SHARED=OFF)	6m 19s
Windows (Win32, win32, Shared, -DDEEPSEA_SHARED=ON)	7m 29s
Mac (Single-Shared, -DDEEPSEA_SINGLE_SHARED=ON)	9m 59s
Linux (Shared, -DDEEPSEA_SHARED=ON)	6m 40s
Windows (x64, win64, Single-Shared, -DDEEPSEA_SINGLE_SHARED=ON)	7m 58s
Linux (Single-Shared, -DDEEPSEA_SINGLE_SHARED=ON)	6m 36s
Windows (x64, win64, Static, -DDEEPSEA_SHARED=OFF)	7m 47s
Windows (x64, win64, Shared, -DDEEPSEA_SHARED=ON)	8m 16s
Windows (x64, win64, Single-Shared, -DDEEPSEA_SINGLE_SHARED=ON)	8m 24s
	1h 47m 46s

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