Support for mixed directionality (only in Grid, so far)

Hi,
I'm trying to add support for bidi and this is what I'm coming up with so far.
The rather repetitive code is due to the inability in CSS to declare something like `float: forward;`/`float: backward;`.
If there was such a value for `float` then the code would be much simpler.

Since there isn't such a value for `float`, the floating direction must be set according to the directionality of the element (and not globally, as it is currently, using `$text-direction`).

If only I could select on the direction *property* of elements! then I wouldn't have to write quadruple code. Instead, I'm selecting on the `dir` *attribute*. Thus the double code - because it considers inheritance.

Does this seem reasonable?

It does work.

It allows mixed directionality HTML with both RTL and LTR grids, automagically, according to the elements' `dir` attribute.

so far, this only includes the removal of the resetting of direction and the support for mixed directionality in Grid. If this method is approved then I'll do the rest of the components.