Anki add-on: True Retention

NOTE: In modern versions of anki (2.1.33 and newer), you ***MUST*** open the Stats window by shift-clicking in order to view addon-based stats information. If you do not hold shift, a different stats window will appear where addon-based stats ***do not display***. This behavior is ***covered in Anki's documention***. Adds a few new lines to the Statistics window, including True Retention (pass rate calculated only among "Review" cards) and a few placebo stats. "True Retention" is a better indicator of learning quality than "Again" rate. Hotfix: Round %age to one decimal point (same as Anki's "correct rate") Update: Added more time periods. Update: Already supports Anki 2.1.x but have to upload a new file for it. Yes, the license of this addon is the same as the code it originates from.