Add a way to inject the global config.

Allows injection of the global ConfigManager instead of having
to get an IdentityManager and call getGlobalConfig...().

Change-Id: Ifa79919e201c418c99899ce5a0a815ac646e5a77
Reviewed-on: http://gerrit.dmdirc.com/2830
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager