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>