mainWindow: add custom match func to completion

GtkEntryCompletion has changed. Now if you call set_text_column
or set the property text_column all manually added renderers will
be removed. Also when you add renderers manually the text renderer
added by set_text_column will be removed and text_column set to -1.

So in order to have custom renderers for the GtkEntryCompletion we
cannot set text_column and instead need to have our own match func.

https://bugzilla.gnome.org/show_bug.cgi?id=722102