Improves handler for DTMF events: when sending several DTMF tones in a short frame, the notification sound is no more clipped but played entirely. The net.java.sip.communicator.service.protocol.maximalRtpDtmfToneDuration property is added to define a maximal duration for the RFC 2833/4733 DTMF tones.

git-svn-id: https://svn.java.net/svn/jitsi~svn/libjitsi@10776 198c5059-ef06-4ec0-9a75-fdd6b0ef00a6