Switch only to MAVLink 2 on first connected link. Fall back to v1 if two links to the same vehicle are connected and one can only do v1

This extension introduces a complete version handshake mechanism which will detect wether the vehicle AND the complete link / routing infrastructure support MAVLink 2. If they do, QGC will switch to the highest protocol version supported by all connected vehicles. If a new vehicle connects, it will re-negotiate the highest possible version. This means that we error on the side of compatibility, which later can be easily changed.

* If new Survey is set to altitude fixed value then use mission default
altitude
* If the user applies a new altitude to the entire mission change
surveys to fixed altitude and apply altitude

This way if the user switches away from manual the altitude is preserved

This reverts commit ac73682a.

This reverts commit 6620f803.

Previously, a completed file upload lead to this assert:
    Q_ASSERT(!_ackTimer.isActive());
because _closeUploadSeesion() was called twice, thus activating the timer
twice.

Plus other ui tweaks

- Added menu options to select new mode as well as new settings and defaults