Handle unreliable PX4 support of BATTERY_STATUS

Change all internal values to double to NaN can be used from unavailable values

Switch to using BATTERY_STATUS for battery info

Switch from builtin video sink to qmlglsink (v2)

Fix video initialization - VideoReceiver did not start because sink was not created because videoBackgroundComponent was not loaded on startup if video was disabled

Allow developers (or advanced users) to override gstreamer debug settings

Checklist enforcing

Remove unused function

Fix Done in Toolbar when switching to Plan View

Allow developers (or advanced users) to override UI settings - quite useful if you set your debug settings in QtCreator and do not depends on QGC settings state (they are reset to default sometimes, for example)

Code documentation and updates

Tweak output.

Exclude QObject and Item. Everything on the c++ side derives from QObject and most things on the QML side derives from Item.

Every single line of this function is commented out, and the
documentation of GStreamer says that it's uneeded.