now the method is aborting early again unless the data is actually used

Sometimes, preparing the data for an event is expensive and only needed
if the event is actually handled. This allows for a quick check before
actually preparing and triggering the event.

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.

The matches our scheme of where_data_what better

The actual draft text got lost when we changed to showing the diff
instead of the full draft.

Specifically, this pull request checks if the current query is already a
valid pageid and then return that as suggested pageid.

Also, tests are added for this method.

Fixes #2355

PS: We may want to somehow better educate/nudge users to use the better
way of creating pages by creating links on existing pages.

and actually use it in the edit form.

This bug was introduced in #2251
c4a386f1

Fixes #2325

Also, for consistency this initializes also $num with 0, while ensuring
that the first FT_SNIPPET_NUMBER results with a $cnt>0 have a snippet.

Punctuation is more natural, makes sense with CSS switched off
and makes screenreaders add a little pause.