Implement new UI

Follow instructions from Design team to implement the new UI: https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/notes/notes-aday.png

Based on those mockups, we can list the following tasks:

• #136 (closed) Implement the list row pattern in main view;
• Implement the list row pattern in tags (notebooks) view;
• Implement the list row pattern in settings view (or remove it);
• Implement context menu in main view;
• Implement start window;
• #154 Implement the adaptive sidebar design;