Simplify setting of spacing at the left/right edges of the toolbar(s)
Simplify setting of spacing at the left/right edges of the toolbar(s)

Rather than modifying the leading/trailing `margin` on the actual toolbar buttons, to achieve appropriate spacing at the left/right edges of the toolbar(s), it seems much more appropriate (and simpler) to just specify an explicit `padding` for the relevant toolbar containers.

Also, for toolbar buttons placed in `splitToolbarButton`-classes we can reduce some complexity around setting the `margin` (since it should always be zero now).

With these changes, we're thus able to get rid of a couple more `!important`-rules.

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