Mnemosyne 2.6.1 released

[Peter Bienstman]

https://mnemosyne-proj.org/download-mnemosyne.php 

- In the grade and easiness statistics, added option to only show active cards,
  as well as options to group multiple subtags.
- In the card browser, added option to filter on cards having all of a certain
  number of tags.
- Fix bug with Mnemosyne not remembering the setting of "Recent cards are cards
  with at most X retention repetitions" (reported by Lenore Raven).
- Fix crash in the review server (reported by Dorian Hawkmoon).
- Fix crash in card browser when having set the card background colour for all
  cards at once (reported by Adam Papai, Jan Simunek).
- Fix interaction between Unicode and Latex (reported by vycska).
- Work around progress bars which don't close under Linux (fix by Emilian
  Mihalache).
- Work around 2038 bug, especially on Android, when grading extremely easy
  cards.
- Fix bug with special characters like apostrophes in export to *.cards format.
- Warn when fields are missing during tsv import (requested by Pavel B.).
- When creating or editing a card, superfluous tags like having both A and A::B
  will be consolidated in a single tag A::B.
- Upgraded to PyQt 5.10.1 and matplotlib 2.2.2 under Windows.