Improve the looks of 2D/3D viewport contextual toolbars

[YuriSizov]

Fixing two annoyances about contextual toolbars:

1. The panel style is visually compressing, making extra toolbars look weird compared to static toolbars.
2. There is always an extra vertical separator at the start of it.

To address the first one I've added expand margins to all sides. I also replaced a control that was used to add extra left padding to the whole toolbar row with a margin container, so this extra padding is added to rows wrapped by the FlowContainer.

To address the second I've added some extra logic to dynamically add separators when we add controls to the toolbar. These automatic separators correctly react to their owner node being hidden and shown too. To make this work, I had to remove hardcoded separators from all custom toolbars, and in some cases rework the layout a bit (for the better).

This does mean that if any user plugins have hardcoded VSeparator, it will look meh, because one would be added by us anyway. But to fix this they would simply need to remove their extra node, et voila, a consistent and neat look!

Some before-and-afters

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[akien-mga]

Thanks!