This adds some simple defaults for the guaranteed placeholders. This
ensures plugins can safely use these placeholders even when a template
did not implement them.

This doesn't really do much except wrapping the functions into a
namespaced class. Autoloading takes care of loading the file.
It's now possible to call lib/exe/ajax.php multiple times in a test
request.

If a key is not set, a PHP Notice is triggered. While these are
currently suppressed, this is still a code smell.

This looks a bit better and is closer to the original, even though the
sizes are a bit weird now.

this got lost in a previous update

There was a bug that the empty string was considered to be invalid if
also a max or min value was defined for a config field of type
numericopt.

also dokuwiki and php are on https

added test coverage

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could be refactored into nicer code once we move to
PHP 7 and get access to the `??` operator.

This makes sure the provided extension ID is a valid id before
initializing it in the extension manager.