use systemwide xdg-open and clean up URL whitelist

desura used to bundle all xdg tools in the binary distribution. But we want to use the system wide installed ones

fixes #531

NOTE: fe8ff527c36ece7384a0329470e3e0932bcc8c63 and 828541ab52122eef548a3d76048d7c4e400bf076 are candidates for the stable branch