Add a SwTextBoxHelper::findTextBoxes() variant that takes an SwNode

This method is called by the ODF export for each paragraph, so limiting
the result for TextBoxes anchored to a single paragraph helps to avoid
iterating over all the frames for each paragraph.

For a test document of 2000 mail merge records and 16 frames, the times for
css::text::MailMerge::execute() are 8m18.811s -> 7m53.575s.

Change-Id: I7a9cd7b23a3e903059ec0ae3a6a8f8309681bb2e