Mission Plan Widget: Added support for MAV_CMD_DO_SEARCH custom pixhawk commands, fixed some tooltip bugs.

Some more bugfixes and improvements for the Mission Plan widget. The "current"-checkbox in "Onboard"-waypoints can no longer be changed by clicking directly, but it is handled internally by QGC. Now there should be one and only one checked checkbox, which shows the last KNOWN "current" waypoint on MAV. The white border around an unchecked checkbox means, that the user is requesting the change of current waypoint, but QGC has not received MISSION_CURRENT message as a confirmation yet. Also,if there is no response from UAV after pushing "Read" or "Refresh", the "Onboard" list will be deleted to indicate, that there is no information available.