Get information about the network

Many users want/need to get information (and updates) about the state of the network(s) to which they're connected.

Examples of such are:

1. Reachability - does the OS say we can reach the internet? The local net? Anything?
   1. Consider also "isHostReachable(QUrl)", if it can be reasonably implemented for most/all supported platforms
2. Media - is it connected by ethernet? WiFi? Mobile network?
   1. Also including the name of the connection (for wifi: SSID, for ethernet/etc.: interface's name)
3. Metered - is the connection metered? How much data is remaining?
4. 'Airplane-mode' - should we notify when airplane mode is enabled?
   1. Some platforms don't have a concept of airplane-mode, so those would reasonably always return 'false'

The final scope of this task depends on QTBUG-87106