Adding contact photo cache

TS is creating a lot of throw-away bitmaps for creating those cropped avatars, even if it's the same avatar all the time. This takes time and if I scroll a conversation really fast, it "hangs" for short periods of time. This is not only because of the bitmaps, but also because of them. This cache should inprove that and reduce CPU overhead.

Also I prepared this for #1027 making it into mainline. If you like this, but dislike #1027, I'll take the (now usused) "colored" flag out.