Labels

Time to explain the labels properly ;-)

This needs review and comments - half the labels is unclear and I bet half of those can and should be removed.

Also, I didn't add all the "v7.x" and "7.0-current" labels yet. What do they mean?

@karlitschek @MorrisJobke 
There is also some backlog milestones in place - how to use this properly? @karlitschek