Add setPath overload to QDeclarativePolylineMapItem

This patch adds a Q_INVOKABLE setPath overload that takes a QGeoPath.
This can be beneficial when a path is already available and should
be set without going through javascript array conversion.

This method should come handy also when pushing polyline data from
C++.

[ChangeLog][QtLocation][MapPolyline] Added setPath overload taking a
QGeoPath.

Change-Id: I6916500473865b8234b48e8df08dfb465a2a75d2
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>