[FIX] fullscreen edit: Use the actual height of the comments box (including it's margin etc) to calculate the height to leave for it (thanks omstefanov ;)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48444 b456876b-0849-0410-b77d-98878d47e9d5