Castero

**Added**
* Added the `default_layout` config setting.

**Fixed**
* Fixed a crash when opening the help page with a very small window.
* Various minor dependency updates.

**Fixed**
* Fixed a crash when adding series with empty copyright or builddate tags.

**Added**
* Added a perspective (accessible by pressing `5`) that displays
episodes from all feeds chronologically.

**Fixed**
* Fixed an issue with parsing timestamps from some (invalid) feeds.
* Fixed an issue with parsing titles/descriptions from some feeds.

**Fixed**
* Fixed reloading feeds not propagating saved playback position.

**Added**
* tistatos - Playback position is saved/restored when restarting the
client.
* tistatos - Created add_only_unplayed_episodes config option for
playing/queueing a feed.
* DGambrinus - Maintain playback speed when playing different episodes.

**Changed**
* To improve performance, the client now (by default) uses an in-memory
database, which is only saved to disk when the client is closed. This
can be disabled with the restrict_memory_usage config option.
* Improved the performance of importing subscriptions (especially those
with many feeds).

**Fixed**
* tistatos - Fixed episode data not being deleted when removing feeds.
* Fixed player volume occasionally not being retained when switching
between episodes.
* Fixed exception with terminals that don't support an invisible cursor
mode.
* Fixed reload operation failing when reloading one feed would fail in a
specific way.

**Added**
* Added a config option for the default playback volume.

**Fixed**
* Fixed new episodes not being properly sorted in the menu.
* Fixed detection for feeds with an empty title tag (they still aren't
allowed, but we explicitly check for this now).