Fix the Phonon notification backend not playing any sound

Phonon won't report its capabilities before a player has been created,
which leads to Quassel thinking that it can't play audio since we checked
for that before instantiating the player.

Some reordering in the ctor fixes this.
Avoid Phonon's deprecation warnings

MediaSource prefers a QUrl now. This shouldn't break with older Phonon
versions, I hope, since it seems MediaSource(QUrl) has been around since
forever.
Enable the ping timeout as soon as the connection to IRC is open.

This fixes the problem where Quassel would get stuck while connecting
if the socket is closed by the server but the core is, for whatever
reason, not notified of this.  This patch works by enabling the ping
timeout as soon as the connection is open, but not actually sending any
PINGs until the IRC authentication is complete.  This will cause the core
to reconnect if at any point it doesn't receive data from the server for
more than the usual ping timeout interval.

Fixes #1249
Add function in bufferinfo.cpp to report if chat messages are accepted

This works around https://github.com/sandsmark/QuasselDroid/issues/98 on
the server side by preventing new buffers from being created if the user
accidentally (or maliciously) attempts to send chat messages in status buffers.
Previously, this would have screwed up the status buffer requiring a manual
DB query to fix.  This should be fixed in Quasseldroid quite soon, but this
patch will just ensure that the core's DB does not get corrupted by older
Quasseldroid clients if the users do not update.
Add core commandline option to require SSL for client connections.

If the --require-ssl commandline option is passed when starting the core,
it will reject any client that does not use SSL.
Add QUrl include for Phonon

Looks like some Qt versions just forward-declare QUrl, so this is
needed.