Running cockatrice in valgrind i didn't found any bad memory leak
(good!), but only a few jumps depending on uninitialized variables.
Even if the possibility of crashes are remote, it's nice to sort them
out.

I should probably bust this out to a method.

Cherry-picked b9cb61abd05f83c67740aafa5e1e6939143547e4

Moved the button from the paths section to be below the "Download high-quality card pictures" checkbox. Button now reads "Reset/Clear Downloaded Pictures". Removed "Path to"s from card/token database, hand/stack/table/player info background, and card back labels.

Added notification messages informing users of the success or failure of clearing downloaded images.

Added a button and the functionality to clear all downloaded images (all images within subfolders at picsPath/downloadedPics) under Settings.

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.

/cockatrice/src/main.cpp:162:16: warning: format string is not a string
literal (potentially insecure) [-Wformat-security]
        qDebug("Could not create " +
settingsCache->getPicsPath().toUtf8() + "/CUSTOM. Will fall back on
default card images.");

Checks if CUSTOM folder was successfully created. If not, prints qDebug message and proceeds as normal except uses default card images instead of custom ones.