Shows horizontal/vertical scroll indicators

* Vehicle selection must be global control
* Disconnect controls must be associated with Vehicle

Made QGCCorePlugin derive from QGCToolBox and expose everything through the same mechanism as everything else.

Also removed intermediate param load retry message

The return lead to data being dropped. Some of this data could have been
part of the ulog header, which lead to corrupt files.

Conflicts:
	src/MissionManager/GeoFenceController.cc

Retry in all the right/possible places. Don’t freak out about things
coming in unknown sequences.

Change in visibility was causing combo box contents to reset. Visible
goes to false even when you move away from Plan view, not just when the
combo is unused.