Add visual button to create new Notebook

[benjo456]

Currently there is no cohesive design language to create new content.

To create a new note, there is a '+' icon. However, no such icon exists to create a new Notebook. Instead, creating a new notebook in the GUI requires a right click on 'Notebooks' and then 'New Notebooks'.

I propose adding a '+' icon next to 'Notebooks' in left side panel to allow for a notebook to be created in one click without looking through menus.