This prevent the visibility change from trashing source setting

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.