Bugfix: tabular row height calculation

The default text format differed from the calculation and the real
typesetting.
Bugfix tabular horizontal alignment

The horizontal alignment should default to »left« for each cell. 
I didn't clear the variable between each cell in a row, thus an
alignment in a cell affects the alignment in the next cell.
Bugfix tabular (no) border collapse

When border-collapse is not used, the shift-up can be nil. A 
comparison later on (nil > 0) makes the program crash.
New horizontal alignment for table cells: justify

The default is now _left_, _justify_ gets the old behavior back.
Well, sort of. The old behavior was totally broken so it was
impossible to rely on a default alignment in a table cell. So in
practice, this should not affect any existing layout.