Use mouse position when zooming in/out

[nicogodet]

When user zoom in or out, it's centered on current view center.

Like in QGIS, it would be nice to use current mouse position to compute next zoom extent instead of using view center.

[rldhont]

Lizmap restricted the map view to the WMS restricted extent. It has to use the current mouse position except if the calculating zoom is out of this restricted extent.