☸ Add option to disable automatic deletion of unsaved autosaves on exit & custom autosave directory

I am writing to address a serious flaw in the current autosave mechanism, which has resulted in permanent data loss on numerous occasions.

Xournal++ currently deletes unsaved autosave files automatically upon closing or exiting the program. Just because a user hasn't explicitly saved a document doesn't mean its contents are deletable or unwanted. Due to unexpected system crashes, accidental shutdowns, or sudden restarts, I have lost countless important notes and documents. These were files I desperately needed days later, only to discover they had been deleted by the program's excessive auto-cleaning mechanism.

Having leftover autosave files accumulate and consume extra disk space is far preferable to losing even a single unrecoverable note. Users can easily delete unwanted temporary files manually, but recovering a permanently deleted document is impossible. I strongly request the addition of a dedicated option: "Disable automatic deletion of unsaved autosave files" (or an option to permanently retain backups of crash/session instances until they are manually deleted).

Furthermore, the default autosave path (~/.cache/xournalpp/autosaves) is too hidden, unintuitive, and imposed on the user. Searching for restored files in this hidden folder causes unnecessary difficulty, confusion, and anxiety. Therefore, please add a "Custom Autosave Path" field, allowing users to specify their preferred folder for automatically saving their files. Giving users direct control over where their temporary backups are saved will make work recovery much smoother and more convenient.