Remove redundant module icon declarations

[fschmenger]

As a consquence of generalizing the Wegue toolbar button implementation in #207, we have to declare the button icon in app-conf as part of the module properties, e.g.

"modules": {
    "wgu-layerlist": {
      "icon": "layers",
      ...
    }
}

The same icon is also used as as the window header icon for the respective module card.
Currently the corresponding window (module-card based) implementations declare a default value for it's icon, e.g. for LayerListWin

props: {
  icon: { type: String, required: false, default: 'layers' }
}

This is redundant and should be removed.

A final question:
Should we should provide a generic fallback for module icons, e.g. https://fonts.google.com/icons?selected=Material+Icons&icon.query=view+module, and make the module icon property optional? Currently the field is mandatory.