Remove top-level menu about jupyterlab_voyager

[zzhangjii]

Hi, @gnestor , thanks for the advice
On planning, I will start the user testing next week, so this top-menu bar serves as an alternative option for user to choose, I will record the user's habits and make a summary.
The ideal design would probably be put functional buttons(save, saveas, undo, redo...) into something like the tool-bar inside notebook:

I'm working on it right now.

And as you mentioned, without the top-menu-bar, voyager-extension is pretty much invisible, plus we do need a place to put voyager-tutorial for users, therefore, I suggest we keep it until the user testing is over.

[gnestor]

Oh ya, toolbar could be very nice 👍

[zzhangjii]

Note, now the top 'Voyager' menu is gone
the tutorial page goes into 'Help';
the "open in voyager" is combined with file context menu;
the "open vl.json in notebook" is added into the vl.json file context menu;
all save/export and show/hide toolbar are inside the context menu of voyager interface

[gnestor]

Thanks @zzhangjii! Much better 👍