tasks #7133: Refactor FullTextSearch system events

Extract system event behaviours in dedicated objects
so we don't polluate plugin code with specifics

Change-Id: Id668ca2c1e21cd9ffd1314c6e314567c7b60979b
Merge commit 'refs/changes/98/2698/5' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7133: Refactor FullTextSearch system events

Change-Id: I7e5146824a7b1d2b3975dcdf653adecb89b9da28
This is 7.2.99.8

Change-Id: I4e02c2de3c7868a68f36409ca4a925780162aa9e