Add 'exists' to QPixmapCache

It would be useful to check to see if a QPixmap exists without having to copy the pixmap.  This is a performance request.  During scrolling, I speculatively read thumbnails that are not displayed.  If the thumbnails have not been loaded into the pixmap cache, I will then load them.

Checking the pixmap includes the cost of copying the pixmap, which I would like to avoid.

static bool exists(const QString &key);