Fix request #4819: [IE8] Triggers in workflow

event.currentTarget does not work on IE. Use prototypejs Event.element()
instead.

Change-Id: I6394fc664b4ae4545081679c2617b1c8d2eac4ae
Merge commit 'refs/changes/41/841/2' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix request #4819: [IE8] Triggers in workflow

Change-Id: I8f242fb934e9a13d42b70c6a53909202b3d7f256
This is Tuleap 6.3.99.24

Change-Id: I176dcec7f78f75d8f0411714a7a90c3d0e39cd9a