Clean up references to MainRootWindow globals

Use showIndicator property to set correct visibility in loader. This leads to correct spacing on all platforms.

Update all files that use a QML Menu item to QtQuick.Controls 2.4 to solve the issue of a menu showing up in random places on Android.

Fixed flight widget position.

Fix toolbar logo visibility

Show the link indicator in the tool bar if two or more links are connected and at least one of them is a high latency link.
Show in the link indicator if a link is inactive.

- 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

This setting enables/disables the LinkIndicator toolbar in which the primary link can be chosen.

The LinkIndicator shows the connected links and lets the user select the priority link,