Option to not publish WMS layers

[JakobMiksch]

• related to Publish added vector layers as WFS in QGIS Server #64
• layers are automatically published by QGIS Server
• sometimes it is wished to not publish a layer (for example when one uses a basemap layer just orientation during project setup)
• it would be great to have an endpoint to chose which layers shall not be published via WMS

some hopefully useful Python snippets:

hidden_wms_layers, ok = QgsProject.instance().readListEntry('WMSRestrictedLayers', '/')
print('hidden_wms_layers', hidden_wms_layers)

# do not show any WMS layers
QgsProject.instance().writeEntry( "WMSRestrictedLayers" , "/", [])
QgsProject.instance().write()

# publish only one WMS layer

# NOTE: the provided layer is the human readable name not the long autogenerated name
QgsProject.instance().writeEntry( "WMSRestrictedLayers" , "/", ['layer-name'])
QgsProject.instance().write()