This is a hotfix update, cherry picking some important fixes from the last 2.4.0-beta.2 nightly release.

Cherry picks:
* Save files in different thread ([#883](https://github.com/LinwoodDev/Butterfly/issues/883))
* Fix delete key not working in markdown mode
* Fix export to multipage PDF leads to empty pages except the current one ([#878](https://github.com/LinwoodDev/Butterfly/issues/878))
* Fix undo/redo not working correctly and freezing the app
* Big translation updates

Read more here: https://linwood.dev/butterfly/2.3.4
