Provide the possibility to switch the encoding used in the text editor

### Summary

In the text viewer the text is currently displayed UTF-8 encoded. As a user it would be nice if UTF-8 is still the default but another encoding could be selected from a list.

### Motivation

If a file with a different encoding, e.g. ISO-8859-1, is stored in a vault and this file is opened in the cryptomator app, it is possible that not all characters will be displayed as intended by the creator of the file.

### Workaround

There are currently two independent workarounds:
1. Create and save files using UTF-8
2. Open these files with a third party app which supports selecting the encoding by clicking on `˅` next to the text file in Cryptomator and choose "Open with..."