Redo checkbox position extraction.

This should be a bit more acurate, as we use the exact position that
PGF knows from placing the object.
Remove compatibility stuff
New checkbox position calculation in tex

This also fixes the coordinate output. It was off by half the
line width in the x direction.
Fix up the y position output of checkboxes.
Fix size of textboxes.

They were all 0.5pt (bp) to small in every direction. This is now fixed.
Remove offinterlineskip and change some spacings.