Make spacing from the edge of the viewport consistent

[GerardasB]

Is your feature request related to a problem? Please describe.
Currently, tool assistance, status bar, and toolbar have inconsistent spacing from the edge of the viewport.

Describe the solution you'd like
Define an internal CSS variable and use it to consistently space the aforementioned UI elements.
Use the same variable to calculate the gaps between button groups and toolbars.

Example:

Red blocks = var(--iui-size-s);
Green blocks = calc(var(--iui-size-s) * 0.5);