mutable really means that the object interface does’t change. Since the return value of isSelected() changes, then _selected is not mutable
clean up the interface (e.g. a function that returns two doubles should return a std::pair)
clean up the interface, fix some static analysis issues
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Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Engine/Curve.cpp
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