Add a validation on area-limit for using the download-layers (adp, adpf), and refer using a local layer

[dieuska]

brdrQ makes it possible to use downloadable layers as a referencelayer (adp, adpf,...). When using very big zone of thematic features this results in downloading this full area of reference-data (long time download and crashing of tool).

Only for small area this download may be used, and when calculating a big area of thematic data, a local referencelayer should be used.