Share context menus between clients

Issue generated from Tuleap's migration script. Originally submitted by: Emmanuel Lepage Vallee (elv13)

This issue track the advancment of the shared context menu implementation designed and discussed in May. The implementation was done in spare time by Stepan and me since May.

* Create the UserActionModel (done, May)
* Define a set of actions for each assets (done, May)
* Allow to define context for assets (done, June)
* Abstract access to assets across all models (done, August)
* Port the UserActionModel to the unified asset access (done, October)
* Move action code from clients into LRC (WIP)
* Add a platform specific extension interface (Unstarted)
* Support tree based actions (Unstarted)
* Support multi selection actions like join calls (WIP, postponed)
* Integration into all clients (Unstarted)