Split Hold event into Hold/Unhold, and move it into core.

(closes issue ASTERISK-21487)
Review: https://reviewboard.asterisk.org/r/2565/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389746 f38db490-d61c-443f-a65b-d21fe96a405b
grr, props.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389748 f38db490-d61c-443f-a65b-d21fe96a405b