 Version 6.15.9

* removed "Set notification buttons" in Preferences as it's not used
* removed "Persistent playback controls" in Preferences as it's not used
* added "Show skip on notification" switch in Preferences, enabling/disabling Skip button on the notification panel
* set reasonable default for switch settings in Preferences
* in Preferences, Notification setting opens system notification settings on Android 8 and above
* fixed combo import dialog layout
* embedded the base class BaseEpisodesFragment in actual classes, and removed base class
* all fragments have action bar and menu in Jetpack Compose
* some class restructuring
