Summarized Changelog:
- Support new GNOME guidelines for the primary menu.
- Sort articles by date by default.
- Make backend plugins share some code and make OPML import work for all backend plugins.
- Make "Oldest First" article sorting work in both Unread and All tabs.
- Improve error message when trying to add a feed that already exists.
- Don't close the currently open article when changing tabs if the new article is on the new tab.
- Use the system default "document" font as the default article font.
- Local and DecSync: Assume RSS feeds are UTF-8 unless they explicitly list a different locale.
- Local and DecSync: Use the RSS feed link to deduplicate feeds, not the link element in the feed.
- TT-RSS: Fix another crash when TT-RSS sends us data with an unexpected types.