WMS layer not correctly concidered by DataSources panel

[Andreas Janz]

Since one of the recent commits, WMS layers opened via Tools > Open WMS Layer aren’t correctly handled by the DataSources panel anymore.

The WMS is shown in the DataSources, but can’t be dragged&dropped onto the map view area.

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[Andreas Janz]

Note that adding the WMS via context menu is still working.

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[Benjamin Jakimow]

resolves #794

→ <<cset cab0b432b4c1>>