Registered Linux MIME type to directly load replays

I adapted our RPM and tested it on @openSUSE with KDE.

![opensuse-kde-openra-mime](https://f.cloud.github.com/assets/756669/2051353/a50908fc-8a87-11e3-8586-606c26a0d4f0.png)

Maybe @obrakmann can help with the Debian package. It should be similar to the spec file changes.
Okay. Decided to just throw, because I don't want to add a dependency to ConfirmationDialogs in OpenRA.Mods.Common.Widgets.