Hamburger button and document menu

[epatters]

We now have enough current or prospective document-level actions that giving each one its own toolbar button would be too cluttered. Instead, we should add a hamburger button to the top-left of the application (where the logo currently is). Clicking this button should pop up a menu for infrequently used document-level actions, such as:

• creating a new document (model, instance, etc)
• duplicating a document (Duplicate an existing model #117)
• import/export document as JSON

Using Kobalte's Menubar component might be helpful here.