[Tracker] Updated todo list

[Sam Thursfield <ssssam gmail com>]

Hi!
Carlos & I spent a bit of time cleaning up this page:
https://wiki.gnome.org/Projects/Tracker/Roadmap

We tried to make sure that all the roadmap items are either simple, or
have detailed descriptions. Nobody seems to have much time to hack on
Tracker at the moment, so it's important that the roadmap still makes
sense in a few years' time!

No doubt there is stuff missing, please point out all the omissions or
add them yourself.

The old, mostly out of date todo items are still available here at the
moment: https://wiki.gnome.org/Projects/Tracker/Roadmap/Old/0.x

Sam