Pass in configuration dir to PluginManager.

Currently we pass in an IdentityManager which it uses to
construct the plugin directory itself, which is odd...

Also keep the reference given to the IdentityManager and
use it in favour of singleton calls.

Change-Id: I1364af35271dbfd7bf42052ebaaabe9f6f764890
Reviewed-on: http://gerrit.dmdirc.com/2674
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager