Physics engine moved into strategy class
Added simple physics engine implementation
Added switching between engines in 2D model ui
Merge branch 'master' of github.com:qreal/qreal into physics

Conflicts:
	plugins/robots/robotsInterpreter/details/d2RobotModel/d2ModelWidget.h
	plugins/robots/robotsInterpreter/details/interpreter.cpp
	plugins/robots/robotsInterpreter/details/interpreter.h
Merge pull request #1207 from dvvrd/physics

Switching between physics engines in 2D model