Pixmap memory, 2nd attempt

This is my second attempt at resolving the problem of card images eating up all the available memory and thus making cockatrice crash.
Cockatrice was messing up with memory when handling card pictures; in memory you would find:
 1. CardInfo's own "pixmap" copy of the full-res picture, that was never freed.
 2. QPIxmapCache's copy of the very same pictures (limited at 200MB)
 3. CardInfo's own scaledPixmapCache, containing a copy of the same image at different resolutions; never freed, too.

This commit removes CardInfo's internal pixmap (number 1. and 3.) and delegates all the storage of pixmaps to QPixmapCache.
QPixmapCache has be instructed to limit the maximum amount of used memory RAM, freeing the least recently used cached card pixmaps.
The QPixmapCache's limit is exposed as an user preference, defaulting to 256 MB.
This could use a docstring informing that it takes MB, and possibly change `value` to `valueMb` or `newMbSize` or something like that