Cleanup "Multi-block Inspector"

Multiblock inspector has several issues.

Often widget is not wide enough and the icons for color/opacity get hidden. Esp. true if block names are long.
When a color/opacity is set, it's background becomes black -- looks quite ugly.
Developer issue: unify codebase with other areas in the UI where composite-data hieararchy is shown e.g. Information Panel, UI for Extract Block filter.

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