some small fixes to layering.css

Changed #layering 'position: fixed' to 'absolute' (it seems not to make
a difference)
- position fixed doesn't work on a couple of devices, especially apple
tablets and phones (due to mobile safari) and is more likely to be
misinterpreted/ignored
- position fixed seems to cause some clients to become unable to
correctly calculate the own height and height of child-elements

layering now has a height set to it, too (100% accordingly to width to
fill the whole parent/body)
- height was not set at all which caused problems in some clients to
correctly set a height to child-elements with a %-height setting

Removed height and width (100%) from .layer as it isn't needed (at least
it seems so)