[BUG] Activity state is not preserved after screen rotation

[Fs00]

I found out that none of the app's activities keeps their state after screen rotation. And that's very bad, because:

• in main activity, if you're inside a notebook you are brought back to the "home" screen
• when you're reading or editing a note, you go back at the beginning, which can be reeeaally frustrating if the note is long

Hope this can be fixed soon.

[Fs00]

PR #154 fixes this problem for main activity. This issue should remain open until we fix it also for note preview and editing activities.