Better label for OK button in Special Characters dialog

Also, added missing colons and changed Help button’s position,
according to the HIG. Didn’t disable resizing here because it’s
actually useful.

Change-Id: I33306eb558f51d3c9889337f4c39465d6c201ff2
Reviewed-on: https://gerrit.libreoffice.org/12143
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>