Support NMEA plugin builds without QtSerialPort

In Qt 6 we have removed the "nmeaSource" property from PositionSource component, and we now suggest to use NMEA plugin if a user needs text files as an input.

However NMEA plugin depends on QtSerialPort. We do support builds of QtSerialPort on mobile platforms, but QtSerialPort is an optional component, and it's likely that the users wouldn't install it for, e.g., Android.

Also, QtSerialPort seems to be unavailable for iOS.

Before Qt 6.5 there is also a bug in Android deployment, which wouldn't deploy the nmea plugin unless the application itself is not linked with QtSerialPort.

The scope of this task is to modify NMEA plugin in such a way, that it would be possible to build it without serialport support. Of course, in this case it would be able to read data only from text files and/or tcp sockets.

While doing this task, we must also consider updating the "Priority" fields of all the plugins, simply to make sure that we load them in the correct order. Because we do not want "NMEA" to become a default plugin on Android or iOS. The plugins that are supposed to be used as defaults should have a higher priority than those that would be used as fallbacks.