DF names shall be globally uniqe within a given card. Selecting by DF
name should be made on card level instead of current DF level.

A new list has been inroduced as a static attribute of MF in order to
track each named DF.

Named DFs can't be tracked in the global namespace of MF, because if we
do so, it is impossible to generate two different cards with the same
content.

By adding a possible self-pointer to MF prettyprint_anything function
got into an infinite loop. Printing of named DFs has been cut off.

Change named_dfs attribute to be a class property