DOCX import: fix handling of dropdown SDT around cell

There were two problems here:

1) The CT_SdtContentCell handlers didn't emit the usual
NS_ooxml::LN_CT_SdtBlock_sdtContent /
NS_ooxml::LN_CT_SdtBlock_sdtEndContent tokens, so the dropdown control
was not created (and then was created with the wrong anchor).

2) In case the SDT was around the cell, the newline character was also
added to the text of the currently selected entry, resulting in an
invalid argument of SwXText::convertToTable(), so no table was created.

Change-Id: I4806626181f40c6d26ff7b25f5dbb863967d8077