Added comment for dummy variable in vehicle.cc.  Enabled _firmwarePlugin call that shows 'Landing aborted.' message.

Allow plugin to define default font size, toolbar height and option to display/hide plan view selector.

Calling delete on a socket callback is not safe and this can crash
QGroundControl. Using deleteLater method from socket to perfom deletion.

previously the start log button was always disabled, even for PX4.

Fixes regression from 9d413108