mapwidgethelper: support tracking of all selected dive sites

MapWidgetHelper::centerOnDiveSite() now checks if more than
one dive sites are selected and finds the most top-left and
bottom-right ones in the coordinate system to form
a rectangle.

It also supports the special cases where a selected dive site
does not have coordinates or the case where only a single dive site
with GPS coordinates are selected.

TODO: implement mapwidget.qml::centerOnRectangle()
This QML function will receive a QGeoCoordinate based rectangle which
has to be centered in the viewport with animation.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>

mobile-widgets/qml/MapWidget.qml

@@ -171,6 +171,10 @@ Item {
 			mapAnimationZoomOut.stop()
 		}
 
+		function centerOnRectangle(topLeft, bottomRight, center) {
+			// TODO
+		}
+
 		function deselectMapLocation() {
 			animateMapZoomOut()
 		}