Clears self referring VclPtr called mpField via FormatterBase in :

1. MetricField, PatternField, NumericField, CurrencyField, DateField,
   TimeField
2. PatternBox, NumericBox, CurrencyBox, DateBox, TimeBox

via respective dispose() methods.

Change-Id: I7f92fefd49756cabe7db7d240781260a2d4cd298
Reviewed-on: https://gerrit.libreoffice.org/18701Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>