bnc#882627: Allow to edit spacing to contents even with no borders visible.

SfxItemSet::MergeValue changes some items from SFX_ITEM_DEFAULT state to
SFX_ITEM_SET which I think is a bug but this patch avoids the problem too.
The issue was: visible changes in some tables, after changing e.g.
borders spacing, because the cells had wrong SfxItemSet after the process.

Change-Id: I676b211e1a4a1d7341c385d63503aa740718ed5d
bnc#882631: keep line visible if proportional line spacing is < 100%.

If line height is smaller than text height, we still want to see whole first
line, so add the difference where necessary.

This also helps to see paragraphs as separate to each other.

Change-Id: I51a87edf0cc03d5b5e130290c90347099a581d4e