remove duplicate plugin code for syntax plugins

This makes Doku_Parser_Mode inherit from DokuWiki_Plugin which allows
for the removal of a bunch of duplicate code form
DokuWiki_Syntax_Plugin. This makes the code easier to maintain and makes
sure all DokuWiki plugins are actual instances of DokuWiki_Plugin.

However this adds a bunch of functions to the "normal" parser modes that
don't need them which could have performance/RAM implications.
reintroduce a tiny bit of duplication

This reads some duplication in the from of haveing a Doku_Parser_Mode
and Doku_Parser_Mode_Plugin class which are basically the same but only
the latter extends DokuWiki_Plugin. This avoids the performance/RAM
problems mentioned in my previous commit. An interface keeps both
logically together.

With PHP 5.4 further deduplication could be done via Traits.
Merge pull request #496 from splitbrain/plugindedup

remove some code duplication from syntax plugins