Clean up some extern decls.

Clean up some extern decls.

Mostly, this hoists extern decls out of .c files and into .h files.
That way, we're more likely to catch errors if the interfaces change.
* alloc.c [USE_GTK]: Include "gtkutil.h" so that we need not
declare xg_mark_data.
* dispextern.h (x_frame_parm_handlers):
* font.h (Qxft):
* lisp.h (Qlexical_binding, Qinternal_interpreter_environment)
(Qextra_light, Qlight, Qsemi_light, Qsemi_bold, Qbold, Qextra_bold)
(Qultra_bold, Qoblique, Qitalic):
Move extern decl here from .c file.
* alloc.c (xg_mark_data) [USE_GTK]:
* doc.c (Qclosure):
* eval.c (Qlexical_binding):
* fns.c (time) [!HAVE_UNISTD_H]:
* gtkutil.c (Qxft, Qnormal, Qextra_light, Qlight, Qsemi_light)
(Qsemi_bold, Qbold, Qextra_bold, Qultra_bold, Qoblique, Qitalic):
* image.c (Vlibrary_cache, QCloaded_from) [HAVE_NTGUI]:
* lread.c (Qinternal_interpreter_environment):
* minibuf.c (Qbuffer):
* process.c (QCfamily, QCfilter):
* widget.c (free_frame_faces):
* xfaces.c (free_frame_menubar) [USE_X_TOOLKIT]:
* xfont.c (x_clear_errors):
* xterm.c (x_frame_parm_handlers):
Remove now-redundant extern decls.
* keyboard.c, keyboard.h (ignore_mouse_drag_p) [USE_GTK || HAVE_NS]:
* xfaces.c (Qultra_light, Qreverse_oblique, Qreverse_italic):
Now static.
* xfaces.c: Remove unnecessary static decls.
* xterm.c (updating_frame): Remove decl of nonexistent object.