FileTypes: redesign video-x-generic

[danirabbit]

In the interest of some of the discussion on #1111 with regards to more standardized file type icons

• Use the same background color as other mime icons
• Replace the filmstrip symbolic with the "play" icon which is less skueomorphic and more closely related to the videos app icon

[waldyrious]

I suppose this will address/resolve #1084.

Also, presumably text-x-ssa would get the same edit, right?

[waldyrious]

more closely related to the videos app icon

For convenience: https://github.com/elementary/videos/tree/master/data/icons

[tintou]

I don't know how to address this, but to me the new version almost looks clickable and not enough like a medium 🤷

Here were my unsolicited 2c