fix oc dialog because the body isn't 100% x 100% in dimensions anymore

Seems to be caused by a CSS change where the body get the width and height (100%) property removed.

cc @jancborchardt 

Tested with Chrom, Firefox and IE8 (9-11 are downloading). For IE8 the innerWidth/height doesn't work and the popup therefore is placed on the top left corner but is fully usable. I don't want to but further code into this to support IE8. Opinions? @DeepDiver1975 @karlitschek @PVince81 ?