Fix Issue #76

These commits add wingo.desktop to $WINGO/data/archlinux/wingo-git and then install wingo.desktop to /usr/share/xsessions/wingo.desktop via the PKGBUILD.  This should be an adequate fix for Issue #76 (https://github.com/BurntSushi/wingo/issues/76) on Arch Linux systems and provide a usable wingo.desktop for other distributions as well.