Move the “check unknown sets” method inside the cards database, so that
it can be executed when the card database gets reloaded after a card
database update.
Additionally, show the user a welcome message the first time they run a
new cockatrice version, so that they know why they get shown the “edit
sets” window and how to hide/disable sets.

 * card: cache simpleName for each card
 * deck editor: remove double comparation of searched term with card
name when sorting

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)..

The check was inverted. LoadStatus::Ok = 0, so only when
db->loadCardDatabase() returns 0 we know the card database path is
correct

I don't know why this even compiled

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

Let oracle add a basic TK set in cards.xml
Unify all the code previously using the string literal “TK”

and save them in cards.xml

We only have a good HQ source for them; any idea on a low-quality
source?

Should fix #412

Cockatrice can now load images both with and without .full. Will need to clean-up bool stripped (used to determine .full suffix inclusion).

I should probably bust this out to a method.

Cherry-picked b9cb61abd05f83c67740aafa5e1e6939143547e4

Like M15’s .png that cockatrice wrongly saved as .jpg
Additionally, move the QImageReader declaration outside of the inner
loop

Removed the QLists for determining image format. Instead, using QImageReader, both when downloading/saving and when loading, to determine the correct format (Cockatrice now supports all QImageReader-supported formats).
Image loading still uses one for loop to iterate through the QList of paths to folders in which to search for images.

Refactored the magic bytes into two QLists: one containing a list of QStrings representing the supported extensions, the other containing a list of QByteArrays representing the magic bytes in hex.
Refactored the image loading loop to be two nested for loops: the outer loop iterating through the QList of paths to folders in which to search for images and the inner loop iterating through the QList of supported extensions.

Switched back to using QFile instead of QImage for saving as QImage's save function adds overhead to the file size, leaving me unsure whether the original image is affected.