Commit b7138e03 (GSoC Writer Table
Styles; Import bugfix, 2016-07-26) changed the SwDoc ctor to always load
the table autoformats, which is expensive for simple documents. Avoid
the load in the ctor by switching to lazy-load and adding a way to count
the number of styles without loading the autoformats when there would be
none.

(mpTableStyles -> m_pTableStyles was only necessary to see if there is
access outside GetTableStyles() to this member, but there were not any.)

Times for 100 hello world inputs: 3863 -> 2753 ms is spent in XHTML-load + ODT
export + close (71% of original).

Change-Id: I6737e7712c775573b56c8b0566e8e7fb615edee6
Reviewed-on: https://gerrit.libreoffice.org/47820Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>