* Unset executable bit from XML and images

I'm not an expert, but those XMLs do not look like any kind of
normally executable scripts. And even if they were -- XML syntax does
not allow shebangs, as far as standard[1] is concerned.

[1]: https://www.w3.org/TR/2008/REC-xml-20081126/#NT-document

* Drop unused global var WINEASIO_PREFIX

It was mis-formatted anyway. Looks like a
copy-paste from wineasio which is broken in the
exact same way[1].

[1] 638ce56a7f/gui/settings.py (L41)

* Unset executable bit

Unset executable bit from all python sources upon installation