Fix resize and change of scroll bar width for Gtk3.

Fix resize and change of scroll bar width for Gtk3.

* configure.in: Add emacsgtkfixed.o to GTK_OBJ if HAVE_GTK3.

* src/emacsgtkfixed.c, src/emacsgtkfixed.h: New files.

* src/gtkutil.c: Include src/emacsgtkfixed.h if HAVE_GTK3.
(int_gtk_range_get_value): Move to the scroll bar part of the file.
(style_changed_cb): Call update_theme_scrollbar_width and call
x_set_scroll_bar_default_width and xg_frame_set_char_size for
all frames.
(xg_create_frame_widgets): Call emacs_fixed_new if HAVE_GTK3 (Bug#8505).
Call gtk_window_set_resizable if HAVE_GTK3.
(x_wm_set_size_hint): Call emacs_fixed_set_min_size with min width
and height if HAVE_GTK3 (Bug#8505).
(scroll_bar_width_for_theme): New variable.
(update_theme_scrollbar_width): New function.
(xg_get_default_scrollbar_width): Move code to
update_theme_scrollbar_width, just return scroll_bar_width_for_theme.
(xg_initialize): Call update_theme_scrollbar_width.

* src/gtkutil.h (xg_get_default_scrollbar_width): Remove argument.

* src/xfns.c (x_set_scroll_bar_default_width): Remove argument to
xg_get_default_scrollbar_width.