Merger of WidgetPlayground

I just applied Nickolay's huge patch that merges the WidgetPlayground
branch into HEAD.  (thanks Nickolay!)

Good stuff :-)
- major rewrite of the main widget.  Structural tags inside span tags
now work; in theory we can now load all well-formed XML files even if
they are "diabolically invalid" - the widget should be fairly sane about
handling the really weird cases, like nesting a chapter inside a table
row inside a footnote inside an anchor etc.
- big simplification of some of the GObject code
- a new plugin that will eventually let us print XML source code from
Conglomerate
- should be possible to figure out the mapping between XML nodes and
coordinates in the widget, allowing scrolling to the selection when it
changes, handling page up/page down etc.
- the design is simpler and hopefully easier for people to figure out
and to hack on

BUT.... the new code isn't quite finished yet...

Bad stuff :-(
- You can crash it by doing certain editing operations that worked with
the old widget
- resizing the widget doesn't yet trigger correct reflows of text, so
resizes look very broken: everything tends to be very "thin"

The main point is that now everything is in HEAD, we will all be working
with the same code, and (I hope) fixing the problems in the new code.

I plan to do another release soonish, giving us a chance to fix the most
painful/obvious bugs with the new code.  If you can, please grab the new
code from CVS (it should hit anonymous CVS tomorrow).

Here's to the future...
Dave