restructured plugin manager

Ignore-this: 4007248aa01f09990612c844c8a83900

This patch moves the different classes of the plugin manager into their own
files and moves formerly global utility functions into the appropriate
class scopes.

darcs-hash:20090725152105-7ad00-89801e811b7eb0d0db25a825d6065aed8ef95c33.gz