Webclient: DOM measurement + CSS Tweaks (#543).

- The JavaScript width calculations involve actual DOM measurement instead of hardcoded pixel constants. As a consequence 1) the Webclient geometry adapts to all browsers automatically, and 2) changing certain CSS paddings/margins do not require adaption of the JavaScrípt.

- All menus have the same font size.

- The page panel has a right padding.

See #543.