Ref #127

It will be saved only if:
* it’s different than the default (the user used a custom url);
* the download actually worked

By default oracle saves cards.xml to the correct path, where cockatrice
will be able to pick it up;
if oracle is not able to determine the path or the user unchecks the
option, oracle will ask for a save path

* add multiverse id to cards.xml
* remove pictures urls from cards.xml
TBD:
* fix/clean oracle’s gui
* add support in cockatrice

Problem: Split card are imported as 2 different cards; need to find
some way to join them

Problem: Split card are imported as 2 different cards; need to find
some way to join them

Use QDesktopServices::StandardLocation for platform independent default paths that don't touch the application's own folder

Added to CardInfo object and to the oracle importer. Next step is the
card info widget.