Drop TeX--XeT code

There is a serious issue with TeX--XeT; right to left “hlist” is reversed
in “hlist_out” to make the text right to left, but this also reverses
“special” nodes and as result all nodes that donate begin/end (e.g.
color or PDF annotation) gets reversed and thus applied in the wrong
order.

To fix this, instead of reversion the node order we should only change
their x coordinates so that they are laid out in the reverse order
visually while still in the correct order in the DVI file (this what
Omega did).

The next commit will use the new approach.
Apply original Knuth and MacKay TeX-XeT

Unlike eTeX’s TeX--XeT, the original code does not do node reversal, it
just inserts “begin_reflect” and “end_reflect” DVI opcode and delegates
actual text reversal to the driver. This shouldn’t be an issue for us
since we require a special DVI driver already.

The driver (xdvipdfmx in our case) does not actually reverse the order
of the text, it instead changes its coordinates, as a result “special”
nodes are kept in their logical order which allows color and annotations
to work correctly.

TODO:
- handling of right to left lines interrupted by displays
- \predisplaydirection?
Fix right to left lines interrupted by displays

Re-instate TeX--XeT fix:
(3) right-to-left text interrupted by a displayed equation is
automatically resumed after that equation
Keep \TeXXetstate but make it do nothing

For backward compatibility.
Allow direction commands in math mode
Fix SyncTeX support after TeX-XeT chnages