Create a combo button+dropdown control, and use it to kill the on-canvas "show / hide inspector", "show / hide code editor", and "show / hide navigator" buttons

[maltenuhn]

Problem
We have an unconventional way to show / hide different panels. Turns out that everyone by default looks in the toolbar, so we should have toggleable menu entries there.

Implementation
This requires a button that can invoke a "context" menu (not actually a context menu, but positioned below the button. #5633 and #5643 have prior art. Unfortunately #5643 is currently on hold, so this will require either a partial solution for that, or a stopgap one otherwise.

Design

• for the button, add another button to the toolbar, left of the zoom indicator. Ask @lankaukk for an icon!
• for the context menu, this needs the three-column pattern (checkmark, label, keyboard shortcut). I don't know if we have this standardised, our react-select popupMenu (which we do not want to use here!) might be prior art.
• Menu entry labels: Navigator, Code Editor, Inspector. (NB: these aren't perfect, but neither are 'left menu' and 'right menu')

(checked indicators like this, plus space for keyboard shortcuts)