Rework QGeoAreaMonitor as a plugin

The QtLocation module has been designed to allow platform-specific backends to be implemented via plugin architecture, nicely decoupling the QtLocation core code base from the backend implementations. However, QGeoAreaMonitor has not been implemented this way. Providing a QGeoAreaMonitor backend (instead of the existing platform-independent implementation) requires the use of #define's based on the platform, and thus maintaining the platform dependent code with the core QtLocation code.

Consider reworking QGeoAreaMonitor to utilize the plugin architecture, or deprecate QGeoAreaMonitor in favour of a more robust GeoMonitoring service (e.g. as proposed in QTBUG-31711)

Note that we found the default, platform-independent, implementation of QGeoAreaMonitor, although demonstrative, not practical in real use. Relying on a constant position update rate wastes power at one proximity extreme, and may not be timely enough at the other extreme. As well, it may not scale well when used to monitor a large number of areas.