Update to use GeoExt 3.1 and OpenLayers 4.3

[geographika]

Update the workshop to use the current GeoExt release.
I'm prepared to complete this if it is thought to be a useful update to the workshop.

[marcjansen]

Yes, please! We should be able to go to

• OpenLayers v4.6.5
• ExtJS v6.2.0
• and the latest GeoExt (unfortunately not a fixed release, a thing we should also change)