Automatic text shaping (experimental)

When setting allocate="auto" on <Output>, the previously
placed objects will be taken into account by letting the text flow around
the allocated area.
Rounding to four digits was a bit too much

An image of width 17 had width 17.0001
Bugfix: change _ to - in sd:reset_alternating()

It is documented with "-", so that's what we should have in the code. We keep
the underscore variant for backward compatibility.
Ugly hack to make things work

This is temporary: One document didn't work properly without this. Need to look into vsplit again.
Whitespace changes and sorted - no code changed
Document 'sd:variable-exists()'
No warning looking for an image w/o file protocol
Allow nx, ny in SetGrid / Grid

You can now choose to set the number of grid cells in x and y direction
instead of choosing the the with and the height. This has the advantage that
the page margins are exact.

Be careful that nx and ny don't make sense in groups.