[Data Views panel] remove "Show background layer" option from context menu

[Andreas Rabe]

Requested by @Fabian Thiel .

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[Andreas Rabe]

@Fabian Thiel the Data Views context menu es exactly the same as the MapView context menu:

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I think we aggree, that we keep 1"Show background layer" option in the MapView context menu.

What I would suggest, is to make it a submenu and show all WMS layers from here:

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