Add Oblivion color scheme from GtkSourceView/Pluma/gEdit

This adds the Oblivion theme from Pluma and other GtkSourceView based editors that at least me and a few people around the web really love. (In my case so much so, that not having the theme was a deal-breaker for me after the Kate update on Debian unstable recently.)

One problem however may be related to licensing: While I have created this theme on my own, I have obviously ended up with the same color values as the original theme and the original theme file is licensed as LGPLv2+. To my knowledge the theme's appearance is not copyrightable as such. And the design produced by applying the theme generally needs to be registered if it is to be protected. The rights afforded by the EU's Community design regulation also allow for unregistered design, but this protection is only afforded for a maximum of 3 years and hence has long since expired if it every existed in the first place. As such I believe that me releasing a “new” theme with very similar design under a different license (MIT) for your benefit is legit, but of course I'm not a lawyer.

I hope this isn't a problem, but I'm mentioning the above for full transparency. Given the above I'm also not sure if it makes sense to mention the original theme creators as “File Copyright Text” in this or the other themes?