[ewiki] new .xpi plugins, reworked plugin hooks [init] and _TEXT check

There is a new "plugins/feature/xpi.php" which allows easy installation
of new plugins (in the also new so called '.xpi' format). It's very
simple to use, and almost all plugins could be packaged into a .xpi file
(two examples ready).
· http://erfurtwiki.sourceforge.net/?XpiPlugins
· http://wikifeatures.wiki.taoriver.net/moin.cgi/AutomaticFeatureInstall

The plugin hook ["page_init"] has gone, and a second stage ["init"]
is now called at top of ewiki_page().
Also the page-flag==_TEXT check has moved behind the ["handler"] calls
together with its ["handler_binary"] plugin call.

mario

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