ENH: Default devices now possible with empty string

Examples/BoneShaving/BoneShavingExample.cpp

Examples/CameraController/CameraControllerExample.cpp

Examples/ObjectController/ObjectControllerExample.cpp

...idBodyVirtualCoupling/RigidBodyVirtualCouplingExample.cpp

Examples/VirtualCoupling/VirtualCouplingExample.cpp

Source/Devices/imstkHapticDeviceClient.cpp

Source/Devices/imstkHapticDeviceClient.h

Source/Devices/imstkHapticDeviceManager.cpp

Source/Devices/imstkHapticDeviceManager.h