X-Plane HIL proven both ways, data conversion and controls feedback set up correctly, needs actual HIL flights tests now

Pushed HIL with X-plane to being close to operational. Receiving from X-Plane  implemented, sending to it is prepared. Needs an iteration of onboard SW first

Erros casused by all of the mergedd branches are now fixed for qgroundcontrol.pro. Working on fixing the errors that now appeare in qgcunittest.pro.

qgcunittest.pro now compiles and runs as long as run in terminal in projcts->Run Settings is unchecked.

Replaced the autopilot-specific inclusion of MAVLink to be more generic. Now if you have no actual QGC modifications to make and just want your messages to be available for viewing/logging you can just create the user_config.pri file and override MAVLINK_CONF.

Modified SerialLink.cc so that it follows the same cross-platform restrictions as the QSerialPort library it relies on.
Windows compilation was broken during 94e91013, which is what prompted this refactoring.

Introduced persistence in RGBDview, needs debugging, fixed a bug in param reading in the simulator, pushed message sender a bit forward, not working yet

Refactored part of MAVLinkProtocol::receiveBytes() as I was trying to debug within it. Nothing major, though I did remove an unnecessary loop.