when param widget is loaded we only want to re-request the whole param list if we don't already have one

Fixed RC calibration trim problem.  Trim was not being set because setTrimPositions() was never being called.

Finished merging QGCVehicleConfig.ui, which I hadn't done properly last night. Things are now in a usable state again.

Removing UASes now returns the UI to an almost pristine state. This was done by altering the activeUASSet signal so that it can emit a NULL UAS. Crashes may still exist in autopilot-specific code.

Removing UASes now returns the UI to an almost pristine state. This was done by altering the activeUASSet signal so that it can emit a NULL UAS. Crashes may still exist in autopilot-specific code.

Change for the new RC calibration widget, still needs an if/else to load the old one for unsupported systems

Set most UI to disabled by default with it only enabled on the addition of an active UAS. Added all missing parameter control buttons to the Vehicle Config widget.

Small code cleanup. If statement was removed because at this point 'mav' has already been assumed to be non-null for a while.

Change for conditionally displayed config items (based on other params), and fix for layout of some config items

Addition of new "User" param for apm.pdef.xml file, allowing for sorting config entries between general and advanced tabs

Variable calibration procedure text based on the autopilot, stored in files/%autopilot%/general/calibration/description.txt.
Added a new QGCTextLabel type for QGW files, which can display MAV text or hardcoded text.
Added tabular configuration screens based on folder structure inside the files/autopilot/airframe folders

Fixed valueChanged bug in QGCParamSlider and added in automatic min/max slider values based on param QVariant type