Automatically add scope attribute in generated table header cells in XHTML renderer

For Dutch Web Guidelines validity a <th> have to contains scope
attribute (scope="col" or scope="row") which means that default wiki
tables are invalid currently.

See http://www.w3.org/TR/xhtml2/mod-tables.html#non-visual-rendering
for example of scope attribute use.

I propose to modify XHTML renderer table header cell to do the following:

• if a scope is explicitly provided reuse it
• if there is no scope:
  • if first line or not first column: <th scope="col">
  • if not first line and first column: <th scope="row">