Add 'updated' property to Root...

because otherwise things like the LineInput have stopped working.
Remove ABC stuff from formatting...

No one outside of us is going to subclass these, so this just makes things
simpler.
Make interface for placeholder population uniform
Remove method dispatcher for style/format population...

as everything should now have the same interface.
Refactor calling to produce string component specifications...

 - This stems from the idea that the thing you get back from a
   Root.format.<name> should actually be a Placeholder, like we had before
 - This means we should be able to cope with using Placholders to specifiy
   Styles with just normal access in the same way that we use for making
   strings with actual content :-)
 - The main thrust of this refactor is to move __call__ from the
   StringComponentSpec to Placholder
 - This required changing the parameterizing specialisation from Spec to
   Placeholder (hence the removal of the ParameterizingStringSpec class and the
   creation of the new ParameterizingFormatPlaceholder class)
 - Unit tests needed to be updated as appropriate, with some that no longer
   apply being removed.
Make PlaceholderGroup immutable, fix addition, add repr
Make escape sequence stack take a default_escape_sequence...

which will be output as a prefix to the output of every 'pop'
Add tests for default formatting, fix usage of default_format and stack
Fix default formats during update and delegation of default formats...

from containers.