Drawing with cairo will be one of the steps required while porting to gtk3.
Following patch needs a bit polish, but the parts I could test seem to be working correctly (I couldn't test i.e. battery plugin without a laptop).
I decided to not touch wnckpanel, as IMHO it shouldn't have made into master yet:
- unloading it segfaults lxpanel
- WnckPanel is not only *not* a box, but not even a container