Add XeLaTeX support to “Watch document”

The idea behind this first implementation is to change just some of the
commands already used for generating pdf documents with `pdflatex`. For this to
work we obviously need to change the typesetting engine from `pdflatex` to
`xelatex`.

Since XeLaTeX usually does not support [partial compilation of the (whole)
preamble][1] we translate an empty pseudo-preamble and then later always
compile the whole document if it is changed.

[1]: http://tex.stackexchange.com/questions/49295/precompile-header-with-xelatex