Fix print composite layer legend

[steff-o]

Fixes #2092 by generating valid legend graphics for Geoserver Layer Groups.

Should generate all rules for all layers in a Layer Group in print legend.

There are some limitations:

1. If a style has several rules, the rules must be named (same as with ordinary WMS layers)
2. A Group Layer must use the default style for each contained layer, i.e. not define another style when used inside the Group Layer, otherwise the map legend and print legend will display different symbols.
3. A top level (without workspace) Group Layer can only contain layers with unique names through all workspaces.

These limitations are all a result of missing information in the response from the getLegendGraphic as JSON request. To overcome that a bunch of Geoserver api request would have to be performed, but that seemed a bit overmuch.

Also thematicStyling is not supported as it not supported by the map legend and would require an issue of its own.

No new configuration is added.

[Grammostola]

Seems to do what it says on the tin. thematicStyling layers with a partial theme list active still print ok. Perhaps if different treatment is needed for similar types of composite layers from other map server vendors a check can be made for the source type (like print-resize does already).

Anyway based on my testing and no linting issues I approve this. Nice addition.