SwXShapesEnumeration: don't generate a set of TextBoxes for each shape

Instead, generate it once and then pass around that list. Again, this
helps ODF export.

For a test document of 2000 mail merge records and 16 frames, the times
for css::text::MailMerge::execute() are 7m53.575s -> 7m30.261s.

Change-Id: Iec993ec34392350b1e26857f7d5434a0a9c494a3