Add podcast subscription and episode play state synchronization

This implements the gpodder API from scratch. It turned out that libmygpo-qt has several critical bugs, and there's no response to pull requests upstream. So using that library was not an option.

The implementation into kasts consists of the following:

• Can sync with gpodder.net or with a nextcloud server that has the nextcloud-gpodder app installed. (This app is mostly API compatible with gpodder.)
• Passwords are stored using qtkeychain. If the keychain is unavailable it will fallback to file.
• It syncs podcast subscriptions and episode play positions, including marking episodes as played. Episodes that have a non-zero play position will be added to the queue automatically.
• It will check for a metered connection before syncing. This is coupled to the allowMeteredFeedUpdates setting.
• Full synchronization can be performed either manually (from the settings page) or through automatic triggers: on startup and/or on feed refresh.
• There is an additional possibility to trigger quick upload-only syncs to make sure that the local changes are immediately uploaded to the server (if the connection allows). This will trigger when subscriptions are added or removed, when the pause/play button is toggled or an episode is marked as played.
• This implements a few safeguards to avoid having multiple feed URLS pointing to the same underlying feed (e.g. http vs https). This solves part of #17 (closed)

Solves #13 (closed)