Add plugin that recenters on user's location

This PR is a port of a plugin written by @lbesson for Camptocamp.org:
https://github.com/c2corg/camptocamp.org/blob/import-carto2013/web/static/js/carto/plugins/MyPosition.js

This new plugin adds a button that tries to recenter on the user's current location if available.

The button icon is a copy of a GIF used in the OL overview map. A better icon is welcome.