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.

- Incorrect signaling causing dirty but to be incorrect
- Remove all not working
- Min fence point count incorrect

Also added new FirmwarePlugin::brandImage support

_finalize methods need information which is not available if all you
have is a mavlink_message_t

before, with a version of eg. 1.5.0, the user was wrongly notified.

Includes ability to load params