New feature: Cancel edit (restores old state)

[korelstar]

In the discussion of #104, a useful feature was mentioned: cancel the edit of a note. This is interesting, because the auto-save feature directly sends all changes to the server. Therefore, the old state has to be stored in memory, and saved again if desired. This action can be triggered using the menu. It is not shown directly in the action bar, because I think it is not often needed. But we can change this very easy.

If the old state is restored, the server would nevertheless increase the modified time of the note by default. Therefore, I'm working on a patch for the server (see nextcloud/notes#4), which allows to set the modified time by the client. Although, this patch is not yet merged (and of course not in a release version), this pull request for the client can be merged already: The changes are fully backward compatible (except for the fact, that restoring an old state will increase the modified time on old servers, but I think we can deal with it)!

Tasks done

• EditNote: New menu item cancel: restores old state of note
• Refactoring of NotesClient: remove recurrent code
• send local modified time to server