Prevent mapbox removing select outlines on every drag operation [1852]

- Reason: `cartesian/select.js.prepSelect` expects
  `dragOptions.plotinfo` to have an `id` property set. Mapbox base plot
  wasn't setting this id property and thus cartesian select always
  cleared existing outlines (user holding Shift key)

Author: rmoestl

src/plots/mapbox/mapbox.js

@@ -388,6 +388,7 @@ proto.updateFx = function(fullLayout) {
             element: self.div,
             gd: gd,
             plotinfo: {
+                id: self.id,
                 xaxis: self.xaxis,
                 yaxis: self.yaxis,
                 fillRangeItems: fillRangeItems