Basic reStructuredText table support
Bugfix: handle empty GroupedFlowable
Table cells should contain flowables, not just text

Breaks the RFIC example
reStructuredText tables: fix row/colspan handling

Row 3 and 4 of the first table in option list.txt reveal a problem with
the row/column spanning code.
Bugfix: last cell in row with rowspan
Recursively prepare() children of GroupedFlowables

This was previously only done for StaticGroupedFlowables and thus
notes nested in a DefinitionList (GroupedFlowables subclass) were
not prepared.
Add level 4 and 5 heading styles for IEEE

Allows rendering of restructuredtext.txt (reStructuredText Markup
Specification).
Size columns of reStructuredText tables

Rows with column-spanned cells cause problems.
Redesign Tabular (now Table) to fit the style sheet system

- Table holds the data (removed TabularData)
- Table is a hierarchy:
    Table > TableHead/TableBody > TableRow > TableCell
- TableCell is a StaticGroupedFlowables

Note: cell borders are not being drawn.
Add copyright/license note to the rst frontend sources
Style the table head cells
Draw table cell borders
More advanced table styling
Handle tables without a head section
Refactor Table.render (split up into functions)
Fix table cell vertical alignment
PDF: number fonts at the document level

- propagate fonts used on page to the top level (~ annotations)

This allows canvases to have no parent (VirtualContainer), if
image placement can adjusted similarly.
Delay adding image XObjects until appending canvas

Canvases can now be parentless (initially).
Allow easy initialization of a COS Dictionary
Suppress display of boxes around links