Rework top bar to not include toolbar
Closes #7679 by creating a definite top bar, and moving the toolbar below it. Still side-steps the E2E test issue by allowing the modeling scene extend under this top bar. I will finally address this in the next step, which is bringing back a proper sidebar that doesn't overlay the modeling scene. I have a fast-follow PR coming that adds visual undo and redo buttons to this top bar, but I wanted to keep them separate.
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