Better building of panes

The aim of this refactoring is to move existing code to dedicated
objects, in order to unclutter MilestoneController which was doing:
- pane info retrieval
- build of the current backlog/planning pane
- its job as a controller

There is no changes from a functionnal point of view, only code that
have been moved, to enforce SRP.

This is a needed step to be able to add new panes for content and
planning management.
Merge commit 'refs/changes/35/435/1' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  Better building of panes
This is Tuleap 6.0.99.5