Simplify getPluginManager calls in the swing UI.

No point calling getMain().getPluginManager() when the controller
has a nice reference you can get at.

Change-Id: I0ed3433cacaa00c08833f456ad5a5f0e65e5ebb8
Reviewed-on: http://gerrit.dmdirc.com/2663
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager