iPad Split Mode

### Tasks
- [ ] Notes List should always be visible on the iPad. Analyze if UISplitViewController is the best approach!
- [ ] The note Editor should have a new button which should allow us to collapse the notes list.
- [ ] Enable Cell Separators on iPad Devices

### Mockups

Start by reviewing the current iPad note-list and editor navigation, then compare the requested behavior with the attached mockups and the proposed UISplitViewController approach. Done means the notes list remains visible, the editor can collapse it, and cell separators are enabled on iPad devices.

Written by the indexing model from the issue text.