bnc#648251: Avoid opening embedded OLE object as "text"

On non-Windows, when double-clicking an embedded OLE object, our glorious
content type detection logic detects it as "Text", and opens it as a separate
Writer document, which is somewhat pointless. (But at least we don't crash, as
we did in the master branch.)

Avoid this by checking for a detected filter of type "Text" explicitly, and
avoiding the fun code paths in that case.

This leads to double-clicks being just ignored. Maybe it would be more useful
to produce a "General OLE Error" message box?

(cherry picked from commit 46ad54725bf28ea75278eb63dbf95c4a29618c1c)

Change-Id: Iae0726b5e9c511a92bdff7229d2978cbf76cb07b
Reviewed-on: https://gerrit.libreoffice.org/11140
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>