last fixes and preparation for release

This fixes some issues in the context menu on Ubuntu which I found in my last-minute testing and adds a 64x64 icon which is required for publishing in the ubuntu store.

It also adds a workaround for the black-on-black text as I'm not entirely sure how long it will take the SDK guys to fix that.

Btw. I'm in the process of creating screenshots for the uploading of the store. In my other apps I usually keep them in the source code repositories. How does that sound to you guys? Maybe in data/screenshots/$platform/ ?