Remove ActionManager's Main reference.

URLBuilder can use a static reference for now, as it makes a lot
more sense than getting a PluginManager from an ActionManager(?!?!).

Eventually we'll need to rewrite URLBuilder to not be a bunch of
static methods, so it can be instansiated with a PluginManager.

Change-Id: I609153d236f82dfd632c7da27d841eb6f031e2dd
Reviewed-on: http://gerrit.dmdirc.com/2664
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager