This adds an event to dbg_deprecated(). This allows plugins to handle
deprecation warnings. One example would be @cosmocode/dokuwiki-plugin-sentry

One thing I don't like, but don't know how to avaoid is that this
function used to abort super early when $conf['allowdebug'] wasn't set.

However for the sentry plugin you probably would want logs, but still do
not show any debugging to end users (which allow debug would do).

So now the backtrace is always built, the event triggered and then
everything is sent to dbglog() which may simply throw everything away.

Suggestions on how to improve this welcome.