Fix errors sometimes caused by QT_PROPERTY

This seems to be a problem of older Qt5 versions.
Add the api_application to the package.xml

Not really sure whether that's necessary...
Pass the API in to the MainWindow
More forward declarations in mainwindow.hpp
Remove the explicit api_application link library

The correct library path here is included in the ${CATKIN_LIBRARIES}.
The manual -lapi_application leads to errors when using cmake directly.
Use actual API data for QuadcopterModel

I can't really test this though as the API methods aren't implemented
properly yet. Besides, I don't have any Crazyflies here.
Wire up the QuadcopterDetailDialog with the API

The dialog now listens to data changes both via the QuadcopterModel and
directly via APIQuadcopter's listener methods. We might want to remove
one of those in the future (i.e. when the listener methods are actually
implemented and work).
Add forward declaration in GlobalCalibrationWidget
Use API data for the calibration dialog images
Do SingleCalibrationWidget's API calls
Wire up MainWindow's buttons

Unfortunately, none of this functionality is implemented in the API yet.
Remove obsolete Qt project file
Implement "Shutdown everything" button
Add Alt keyboard shortcuts for all buttons/actions
Merge branch 'integrate_irrlicht' into 3dtest

Conflicts:
	CMakeLists.txt
	GUI-Application.pro
	cameramodel.hpp
	mainwindow.cpp
	mainwindow.hpp
	mainwindow.ui
Added classes for displaying of objects
Merge branch '3dtest' into integrate_irrlicht

Conflicts:
	GUI-Application.pro