Harden FTPManager::download against packet loss

LinkInterface and Vehicle uses mutexes to synchronize writing bytes to the link instead of queued signals

Added a transmitOnly flag to UDP config. When enabled, the socket receive buffer is set to zero such that no data is ever received. Also updated the MavlinkSettings page such that new config settings respect the setting visibility.

Updated mavlink forwarding implementation to forward all mavlink messages received on all channels onto a single UDP port. A new mavlink forwarding UDP link is created if forwarding is enabled. The mavlink settings page now has a Fact checkbox for enabling forwarding and a fact text field for specifying the host name.

Added mavlink forwarding as an option to UDP connections. Enabling this setting will cause every mavlink packet received by QGC to be forwarded on the link, with the exception of packets originating from said link.

App messages now use simple modal message dialogs. Vehicle messages continue to use non-modal popup.

To have both order from the GCS and drone messages in logs
So that it's possible to analyze the complete exchange between QGC and the
drone.

- Add a signal emitted by writeBytes of each LinkInterface
- Connect this signal to MAVLinkProtocol::logSentBytes in LinkManager
- MAVLinkProtocol::logSentBytes will parse and log messages sent from
  QGC

qobject_cast is times faster dynamic_cast for objects that
inherit QObject. also, use auto keyword to not duplicate
class names.

New qml base log replay status bar

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

USe correct number of available channels

Fix broken NO_SERIAL_LINK build

On desktop (Ethernet connection), the ground unit pushes UDP directly, which makes the use of an actual "Link" not possible. Instead, I've moved all of Taisync support to a "manager" owned by QGCToolbox.
Added a Taisync settings with some basics already wired and running.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

- Improve function names
- For notifications of an active link change only show the priority keyword if multiple links are connected
- Use QMap instead of QList for the HeartbeatTimers
- Style improvements
- Simplify sending the MAV_CMD_CONTROL_HIGH_LATENCY command

Add the HeartbeatTimer class to track a single heartbeat for a single link.
For every new received heartbeat an instance of the class is created.
Every instance emits a signal if it timed out or the timer is restarted.
Each vehicle then decides based on all different signals which link is the
priority link.

This ensures no ports are being displayed to the user that are definitely unusable.

We were connecting to system peripherals before, which adds boot delay as QGC re-tries opening the port. We were also opening serial ports two times on Mac OS: Once as /dev/cu.usbmodem1 and once as /dev/tty.usbmodem1 - since the cu.usbmodem alias was already open, the 2nd opening always failed, leading to further boot delay.