rcp: Allow updating RCP installations

rcp: Allow updating RCP installations

* Add the nightly build update site to the default repositories
* Add the "Check for Updates" command to the Help menu
* Restore the -SNAPSHOT suffix of the rcp.product/pom.xml version, to
  allow updating the RCP feature between different snapshots.

Change-Id: I718d37870a21d7238dc58e8a319fc56e2f1d63f3
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/39623
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>