Draft: added recently used fonts feature

This change includes the definition of a structure Recently_Used_Fonts in the FontLister class. This is a combination of a list and a set. The name of the set is kept 'dupe_check' intentionally because it indicates the purpose of using a set here(To check for duplicates). Position wise, the recently used fonts get inserted right after the fonts used in the document. By default the maximum size of the recently used font list is set to 10. In future, I'm planning to enable to user to modify this size based on his/her needs and store this preference in the config file. Also, the styles of the fonts in the recently_used list are still to be added.