Move panel buttons to the top center

[emilk]

These should be in the center of the top bar.

This will leave space on the top right for:

• Maximize button in the web viewer (Fullscreen mode on web #6433)
• Close/minimize/maximize buttons on Windows (Custom window frames for Windows and Linux #1063)

It will also just look better.

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Currently the top center is free real estate, except that it is where we put the "Debug build!" warning text. We can put that somewhere else though (e.g. to the right of the panel buttons)

[emilk]

Alternatively: we just move them left enough to fit the buttons we want to the right of them

[emilk]

Let's save the top-center real estate for something fun in the future (e.g. a global filter/search)