rfc: drop xwt.lib.abstract

The xwt.lib.abstract namespace was there to provide skeletal code for 
the major widgets / -ables in the event that a theme would require 
uncommon keybindings / -able-integration.

Nitrogen now gives the option to provide a reference to a trap, so it 
can be removed from outside the scope of a template.  This means that 
any of the behaviour that might differ on a few rare platforms can be 
referenced and removed if needed.  This leaves no compelling reason to 
keep xwt.lib.abstract as a namespace for the separation of lib code on 
singular templates.

If it is desirable to separate the widget lib code and the -ables then 
we could have the -ables in xwt.lib.abstract, although that's not 
something I particularly care for.

So, unless somebody objects, all xwt.lib.* and xwt.lib.abstract.* files 
are being merged and xwt.lib.abstract.* is going.

This is something that David and Adam both wanted originally anyway, so 
it further unifies the direction in which we all want the widgets to go.

- Charlie