Add option for GUI camera to show map view instead

[doc]

As per the subject; the built-in trains interface allows us to switch between camera and map views for all the trains, but there doesn't seem to be any way to do this when we use cameras ourselves. I've tried zooming out a long way with the camera but it never turns into a map and starts causing performance problems due to the amount of stuff being rendered (whilst not really being very clear at all).

I'm using the camera in my Portals mod so you can see the portal on the other side of the link when you hover, unfortunately it's not always very clear without a map view where that portal actually is.

[kumpu]

+1. That would be very useful.

[Rseding91]

I've added a CustomMinimap gui element type for the next version of 0.16.

[eradicator]

While you're adding view widgets (yay ), how are the chances for my request for exposing the single-entity-preview?
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