'Library.get_previewimage' uses 'nx_common.default_fanart'

*I'm submitting a ...*
- [x] bug report
- [ ] feature request
- [ ] support request

## General infomration

### Prerequisites

* [x] Are you running the latest version?

### Description
The function `lib.resources.Library.get_previewimage` uses the variable `default_fanart`; this exists in `KodiHelper` but not in `NetflixCommon`. A possible solution could be to write `self.nx_common.get_addon_info('fanart')` or `self.kodi_helper.default_fanart` instead (I tested the first).

### Steps to Reproduce
Open **Exported** with existing entries.

Start at lib.resources.Library.get_previewimage and inspect how it obtains the default fanart value from NetflixCommon and KodiHelper. Reproduce the issue by opening Exported with existing entries, then verify that preview images load without the missing-variable error.

Written by the indexing model from the issue text.