Workaround the two remaining issues, mostly fixing the “load deck”
screen in tab_game:
 * Only preload up to the first 300 cards of a deck;
 * don’t even bother trying to load card images if they’ll be shown
with width <= 10

Trying a 17k cards deck like the one proposed in #67 is still a no go,
mostly due to the cpu time used to create all the objects. Anyway, i
tested decks up to 8k cards and they works fine now (if you like to
wait while trice is loading them)..

+ Users can now enable/disable the mouse over card scaling.

+ refactored code
+ improved card border lines (black)
+ cards no longer have curved edges, this prevents card image corners
showing behind.
+ tapped cards now have better border lines

Drop CardInfo’s own pixmap caches; use QPixmapCache instead.
Use QPixmap references instead of pointers.
Being stored in QPixmapCache, all card images are now subjected to
QPixmapCache’s size limit

Cards now scale up by 10% when mouse over.