Support the "medium" font weight

Support the "medium" font weight

* lisp/faces.el (set-face-attribute): Mention new font weights in
doc string.
* src/font.c (struct table_entry): Allow more synonyms.
(weight_table): Expand to support separating medium and normal
weights.  Also add heavy/ultra-heavy and some other variants.
(font_parse_fcname): Support more names.

* src/gtkutil.c (xg_weight_to_symbol): Support all the Pango weights.
(xg_style_to_symbol): Make into functions.
(xg_get_font): Adjust.

* src/w32font.c (w32_to_fc_weight): Use symbols.

* src/xfaces.c (syms_of_xfaces): Add the new symbols.