Support user provided card themes

[Tomin1]

Currently Patience Deck supports only one set of card themes, all derived from Anglo theme in GNOME Aisleriot. However Aisleriot provides many more card themes that could be used and some users might also want to create their own it would be nice to be able to utilize those. Since it's not always clear if cards can render properly with QtSvg and adding many more styles would be UI design challenge, I think a good middle ground would be to support user provided card themes. Basically user could put their card theme to ~/.local/share/site.tomin.apps/PatienceDeck and then the UI can display an option to select a file from that directory which would open file listing. This would also allow testing different card graphics for future inclusion.

Patience Deck has one limitation for card decks that Aisleriot doesn't have: the cards must be in hard-coded aspect ratio. This could be improved so that aspect ratio is calculated from the SVG file, although it could be a challenge for table layout calculations but as long as the built-in theme works well that should be fine.