As it is wrong (says Thorsten) to override the methods from
XFastContextHandler in ContextHandler2 in its derived classes, mark them with
SAL_FINAL to catch that (when compiling with a compiler thaty supports
"final").

Do corresponding changes then; change createFastChildContext() methods to
onCreateContext() ones instead.

Now the unit tests work, and a sample PPTX that uses embedded OLE thingies
loads without crash, but unfortunately it looks like crap...

Change-Id: Ie30022f37418ba9caf8ce59fe4003b808fa900f3
Reviewed-on: https://gerrit.libreoffice.org/4578Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>