Improve area monitoring API.

1.) QGeoAreaMonitor renamed to QGeoAreaMonitorSource
2.) Add new QGeoAreaMonitorInfo data type encpsulating individual areas to be monitored
3.) Port positionpoll plug-in to new features
4.) Make positionpoll monitor thread safe
4.) Extend and fix the QGeoAreaMonitor unit test
5.) Fix documentation.

Task-number: QTBUG-31711

Change-Id: Icfc982de4753d2f43cb4d15c234eb7b7c039a0c4
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>