GeoData: trigger visibility of documents from the theme legend

For this purpose, a property attribute is brought in the following:
- GeoSceneGeoData (aka geodata layer)
- FileManager and FileLoader
- GeoDataDocument
Then MarbleModel reacts to mapTheme settings signals and toggles
GeoDataDocument visibility.
The property name is generic and simply needs to be coordinated in
the dgml file in the following sections:
- <layer><geodata property="rivers">
- <settings><property name="rivers">
- <legend><section checkable="true" connect="rivers">