Prepare next development iteration

Change-Id: Ib175411b9d6fc460e4276470e133eb99ccccffde
MediaSet: Do not erase element from children map before unref its children

Change-Id: Ic06878429859fd12855beb9fd4625ff6a874eb2a
Send sessionId on events

Change-Id: Ic27d433d60955bf549867d3ea5cdc6608ccff2a1
HttpEPServer: Add Access-Control-Allow headers on all requests

Change-Id: If064c8561894e53c4d3837b5d1da19e9bd318a7c
Modify management of event handlers

Change-Id: I56cc09a0e2eb4e0fefcfa6c3e12b3efc43ba9241
Update plugins to last release version

Change-Id: I09a40613de8eb876b78c7568214823fca59a9cca
Prepare release 4.2.1

Change-Id: I2aa19049325e3a2d092d175b852b1fb8a510972e