Delayed approval mechanism

This patch introduces the possibility for a delayed approval to be sent
back to Gerrit. This is a relatively advanced use case, so the setting
has been placed in the Advanced section for the Trigger settings.
When one selects "delayed approval", the completion of the build will
NOT send an approval back to Gerrit. Instead, the triggering event stays
open, and needs to be closed later on. This can be done by using the new
post-build action "Send a Gerrit delayed approval". At the moment, this
notifier expects to read the name of the job and build number from build
variables. If these point to an existing build which has a Gerrit event
as a trigger, which has not been submitted due to "delayed approval",
then that approval will be sent. In particular, this allows to modify
the status of that build through other job (e.g., using Groovy scripts),
and mark it UNSTABLE (e.g., if a subsequent build were to fail), which
is a way one can change the result of the Approval in Gerrit.