Add # to FormulaControl in xaml

[alonfnt]

Currently it is not possible to add a formula containing a #. In latex it works by using # but that is not possible in xaml.

If you could point me out to a direction where to change this I'll gladly send a PR.

[ForNeVeR]

For now, please use \text{#} as a workaround.

To fix the issue, I think that modification of TexSymbols.xml and maybe DefaultTexFont.xml will be required: you need to match this character with some TeX font symbol.

Note: in LaTeX, only \# seems to work because raw # is some kind of macro character.