Where commands are executed globally, pass in a GlobalWindow.
Make the GlobalWindow a singleton that persists, and is simply
added/removed from the WindowManager when the user wants to
show/hide it.
Change-Id: Iac94848487442509bdbf8ec98f67983e313ed743
Reviewed-on: http://gerrit.dmdirc.com/3451
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>