Add titlebar / menubar

[justvanrossum]

This is work in progress: #929

But we're a bit stuck in defining the exact requirements and contents. This needs to be discussed an determined.

[fatih-erikli]

In future we may want to implement something like "Open recent" in visual studio code, or "Recently closed tabs" in Firefox. Multiple-levels menu should be implemented in MenuPanel in order to have that functionalty. We will need a different ticket for that.