Remove static ThemeManager methods.

Tidy AddonToggle's constructor a bit so it's less pathological.

Change-Id: Ief53bb0398c73cba5f718a33c7cb0089b3fdbf9d
Reviewed-on: http://gerrit.dmdirc.com/2745
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>