Add 'mapable' mixin

[chrismayer]

Instead of binding the OpenLayers map manually to every component, which needs a reference to it it would be useful to encapsulate this in a mixin. The mixin could be named like 'mapable' and has to do the following:

Listen to the ol-map-mounted event of the event-bus and bind the map as member variable of the component:

      WguEventBus.$on('ol-map-mounted', (olMap) => {
        // make the OL map accesible in this component
        me.map = olMap;
      });