A revision to the Widget manual

A revision to the Widget manual

* doc/misc/widget.texi (Widgets Basics, Working with Widgets)
(Widgets and the Buffer, Widget Gallery, Customization): New chapters.
(Basic Types, Sexp Types): Demote to subsections.
(Widget Browser): Rename to Inspecting Widgets.
(Widget Properties): Remove.

(Top): Adapt menu to changes.
(Introduction): Rearrange text.  Move warnings to a better place, and
user options to the Customization chapter.
(User Interface): Don't fully describe commands and customization
options here.
(Setting Up the Buffer): Expand on widget creation process and add
documentation for useful functions which deal with
creation/conversion.
(Defining New Widgets): Expand the documentation on define-widget.
All relevant properties moved to the description of the default
widget, in Widget Gallery.
(Utilities): Add some more useful functions not previously documented.
(Wishlist): Remove out-of-date items.