Substantially bugfixed and improved deactivation/activation and persistence of widgets. A lot of testing of Google Earth view, only minor improvements

Added to 3D view user switching between local and global coordinate system. Fixed bug in WaypointView: new instance used to assume frame is global.

Fixed imagery rendering issues. Robot position in Pixhawk3DWidget is now derived from the global pose (lat/lon/alt) instead of the local pose.

Fixed image cache bug: when a cache is full, a new image replaces the first cached image instead of the oldest cached image.

Added command to select target position for robot - command can be accessed via a right-click in the 3D View perspective --Lionel