QtHelp: Check the results when adding or removing custom filters

removeCustomFilter() ignored the results of both DELETE statements and
returned true even when they failed. addCustomFilter() ignored the
results of adding new filter attributes and of removing the old
attributes of the filter. Neither used a transaction, so a failure
could leave a filter partially removed or updated.

Check the results, run the changes in a transaction and check the
result of the commit.

Task-number: QTBUG-150686
Pick-to: 6.12 6.8
Change-Id: I0969a3efaa010fc414f1d99c8c5efd66af63affd
Reviewed-by: Kai Köhne <kai.koehne@qt.io>