XPath tests

Some more XPath tests before we change the XPath
parser.
Group specific grid

Groups used to have the same grid as the current
page on which they are defined. Now the grid
defaults to the current page's grid, but
it can be set with the known <Grid> command.
Bugfix: Columns in tables are ignored
Bugfix: color in colorbars are not local

colorstack doesn't work left and right of a rule.
We now use pdf_literal q...Q.