[css-page-floats] First cleanup

Hey,
I have spent the last few weeks looking at the manuals of PrinceXML, the
Antennahouse Formatter, Håkon Wium Lie's CSS Figure Spec [1] and have
discussed with the Vivliostyle team what they would need from a page float
spec.

I have tried to add the most important points to the CSS Page Float spec
draft [2].

The most important points are likely:

-- Option to specify flow relative directions
-- Addition of a float-reference property that allows inline floating
(current right/left floats), as well as floats within columns, regions and
pages
-- Removal of advanced features that are mainly used in very specific
situations

All of these points still have to be elaborated and exemplified quite a bit
more, but with this draft I think there may already be some interest in
discussing whether this is going in the expected direction and whether
there are other major issues that are not already mentioned in the document
that need to be taken into consideration at this stage.

[1] https://figures.spec.whatwg.org/
[2] http://dev.w3.org/csswg/css-page-floats/

--
Johannes Wilm