DokuWiki typically instantiates a plugin once and reuses
that instance whenever the plugin is required. However on
some occasions DokuWiki will request a new instantiation of
a plugin.  This particularly applies to render plugins.  This
patch allows a plugin to force DokuWiki to reuse the existing
instance.

If a plugin wishes to only be instantiated once then it
should implement an "isSingleton()" method and that
method should return boolean true.

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