Display icons with tooltip instead of text for counters in status bar

It reduces blinking effect by fixing width of elements.
Also i modified the counter of pending requests to reflect network activity better, including "active" requests in the count (so the counter displays the max of queued requests and active requests). It solves an issue with long requests (ie. cover art downloads), where counter was 0 while data was still downloading.

I re-used existing icons, until someone provide better ones.

![status_icons](https://f.cloud.github.com/assets/151042/717142/8228eb74-df45-11e2-887d-484ee09a94c7.png)