Tidy up some TabCompleter usages.

Change-Id: I2eb5ac89568b524ef13cf1d455eb5b51904c806c
Depends-On: I1d61d1895025f3b7febd7cae1e8411d0b198f141
Reviewed-on: http://gerrit.dmdirc.com/2866
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager