* [#215] Handle metadata migration
Makes it possible to change "metadata keys" and the "app metadata key prefix" without losing data
* [#215] Fix checking and handling missing reader settings in metadata
With the new migration logic missing reader settings couldn't be detected anymore in case the metadata included outdated app metadata keys for the reader settings.
* [#215] Fix name of "IReaderSettings" property
* [#214] Move "reader settings" util functions into new file
* [#214] Extract "options" of "ReaderSettings" to new component
* [#214] Add component for changing the default reader settings
* [#214] Add default "ReaderSettings" to "Settings"
* [#214] Use default "ReaderSettings" stored on the server
Wait for both "manga" and "defaultSettings" to be loaded.
Otherwise, by the time the "manga" response was received, the "defaultSettings" might be still loading and thus, the local fallback "default" settings will get used as the "default settings"
* [#214] Save and update "ReaderSettings" options in case they are missing
- Manga gets read the first time => save settings in metadata
- Default settings get opened the first time => save settings in global metadata
