Added gui plugin documenation and screenshots

etc/doxygen/images/HandUnitGUI.png

etc/doxygen/images/KinematicUnitGUI.png

etc/doxygen/images/PlatformUnitGUI.png

etc/doxygen/images/Plotter.png

etc/doxygen/images/RobotViewerGUI.png

etc/doxygen/pages/RobotApi-Gui-plugins.dox

...e/RobotAPI/gui_plugins/HandUnitPlugin/HandUnitGuiPlugin.h

...botAPI/gui_plugins/HapticUnitPlugin/HapticUnitGuiPlugin.h

.../gui_plugins/KinematicUnitPlugin/KinematicUnitGuiPlugin.h

...PI/gui_plugins/PlatformUnitPlugin/PlatformUnitGuiPlugin.h

...tAPI/gui_plugins/RobotViewerPlugin/RobotViewerGuiPlugin.h

...ugins/SensorActorWidgetsPlugin/SensorActorWidgetsPlugin.h