Show icon if a note is not synchronized (hint to a possible error)

[korelstar]

This pull request introduces a new icon which is shown on the right margin of a Note in the NoteListViewActivity if (and only if) the note is not synchronized, i.e., the note is new or was edited and the respective data is only local and not yet on the server side. This gives a hint to check, if there is a problem with the synchronization (partly closes #119). Until now, if synchronization fails, one can't realize this and somebody may think everything is fine.

The implementation introduces the new model DBNote which extends Note with the status attribute in order to display the status of the note. The icons come from the original android settings app: https://github.com/android/platform_packages_apps_settings

[stefan-niedermann]

hm, okay, i'll have to check and test this locally, could take some time, sorry.
PS: could you please use the material icon instead of the holo icon for consistent user experience? thanks. (search https://materialdesignicons.com/ for "sync" and you'll find it)

[korelstar]

Oh, that's a nice website. Didn't know it. I've updated the icons now.
I'm using this branch as base to work on #117, but this will still take some evenings :-)

[korelstar]

Okay, my fix for #117 is nearly finished, so all changes done here will be in the next pull request. So I close this one, since the refactoring done here was not perfect. However, comments are welcome and feedback will be integrated into the upcoming pull request.