Added functionality to export individual podcast episodes

[uiskla]

I have added functionality to export individual podcast episodes to a storage location choosable by the user.

Use cases for that:

• The user might want to send podcasts to a wearable (eg. sports tracker with MP3 playback functionality).
• The user might want to archive podcast episodes for later reference.
• The user might want to share the actual postcast and not only the link with others.

The patch extends the app like this:

• The settings have been extended to choose a location for the directories and files.
• The settings have been extended to select that exported episodes are automatically marked as played.
• There is an additional Save button for each episode (under more) that saves the episode under the path <user_select_dir>/<podcast_name>/<episode_title>.mp3. The path elements coming from the podcast are sanitzed to contain space, letters, digits, and a limited number of special characters like "-".

The functionality has currently been only tested on Android. The iOS build fails in the master branch, so I could not test it.

I needed that mostly for exporting podcast episodes to send to my Amazfit sport watch for listening in the gym. An alternative implementation for that would be to just give an option on Android to save downloaded podcasts not in private app space, but on some location accessible by the user. The workflow would then be to just copy episodes from there to the wearable and mark them as played. On iOS one could make the app's directory appear in the files app.

Please let me know what you think of my solution, or if you would prefer the alternative I have outlined in the last paragraph or if you don't like this idea at all.