Implements
https://bugs.dokuwiki.org/index.php?do=details&task_id=2525

In an older version of PHP a file_exists() call would issue a warning
when the file did not exist. This was fixed in later PHP releases. Since
we require PHP 5.3 now, there's no need to supress any error here
anymore. This might even give a minor performance boost.

In stead use the type as provided to the function. Fixes #920

many PHPDocs
some unused variables
some dynamically declared variables declared

This introduces a new dbg_deprecated() function which allows for easy
marking of deprecated functions. Each call is logged to the debuglog
when debuggin is enabled.

When page is removed, and diff can be requested between a revision and
current situation. This results in right revision is 0.

Similar case just after creating a page. A diff between the first
version and nothing before, result in left revision is 0.

In these cases a empty dummy revision is placed as selected value in
dropdown. Otherwise user got distracted by the revisions details shown
in select field,  which are not related to the diff below.

With unit tests

One case is not yet fixed:
when rev1 is first rev from changelog, only $max/2 revisions are
retrieved, but it should retrieve $max revisions.

remove empty rev from ml()
rename getProperRevision() to getLastRevisionAt()
make getLastRevisionAt() a method of ChangeLog

changed variable name $create_time to $modified_time

Also isset on meta-last_change-date entry.

 Navigation links to move to previous edit before revision in left column or next/last edit after revision in right column.

when the SKIP_DELETED flag was set, no recent changes where returned for
media only queries, becuase file checks where done on page files instead
of media files

When an overlong edit summary was given for an edit, the resulting
changelog line could be longer than the chunk that is handled in the
changelog reader (8192 bytes) causing the reader to abort the operation.

This meant that old revisions where no longer accessible. This patch
fixes the reader (it continues reading chunks until a full line is
found).

However, limiting the summary makes sense and will be added in another
patch.

Before it was possible that changes for media files the user can't
access were shown if the user could access a page with the same name.
This also reverts the file existence check so it now really checks for
media changes that the media file exists and for page changes the page
and that not one of the two suffice for both.