DOCX import: improve table style handling wrt. cell borders

The symptom was that some cell borders were missing. It's because in
Word, cell borders are additions to table borders, so if a table border
is single, but the cell border is none, then the outcome should be
single, not none. In Writer, this is a single UNO property, so if it's
set to SOLID then to NONE, the latter wins.

There are two situations where we now do the right thing:

1) style-cell-border is set, direct-cell-border is none -> outcome is
now inheriting (style-table-border, direct-table-border, etc.)

2) style-cell-border is none, direct-cell-border is none, but
direct-table-border is set -> outcome is now direct-table-border.

Change-Id: I320ae908c61221c8020e3b5323c31dec11c15b2f