Introduce a TabCompleterFactory.

Make Servers use it to break their dependency on AliasWrapper,
which was odd and annoying.

Change-Id: I1c515b65d88049bd9288e1334e5ebe5fa9c31a80
Reviewed-on: http://gerrit.dmdirc.com/2792
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>