Add NoteIndexStorage class.
This is used for saving and loading the note index data using nedb.
Save the note index via nedb.
Merge branch 'feature/Use_nedb_for_storing_note_index' into develop
Enables to delete notes by pressing delete key.
Enables to delete a note by clicking the delete button in the note list pane.
Merge branch 'feature/note_deletion' into develop