Replaced the GIF spinners with 24-bit MNG animation

So the gif ajax loaders are not looking too good right now, so I replaced them with more smooth MNG animated files which are 24 bit, and fully supported by Qt. I managed to find loaders which fit the rest of the graphics in communi-desktop much better than the current ones.

There's a bug with the loader not spinning (not just in my revision, even right now, with gifs) when the dark theme is active. My Qt is quite old, 2.6.3, so it may be an old bug or something.

Thank you.