Fix some problems with moving and resizing child frames

Fix some problems with moving and resizing child frames

(1) Provide new option 'x-gtk-resize-child-frames' which allows
to either hide a child frame during resizing or asks GTK to
resize it "immediately".  This is needed because desktops like
GNOME shell otherwise won't allow resizing child frames at all.
(2) Do not try to synchronize the position of a child frame
after moving it.  Needed because the present implementation
introduces a 0.5 secs delay which makes dragging child frames
virtually impossible with Lucid and Motif toolkits on desktops
like GNOME shell that use invisible outer frame borders.

For further information see the thread starting with
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00343.html

* src/frame.c (syms_of_frame): New symbol Qxg_frame_set_char_size_4.
* src/gtkutil.c (xg_frame_set_char_size): Hide child frame
during resizing when 'x-gtk-resize-child-frames' equals 'hide'.
* src/xfns.c (x_set_parent_frame, Fx_create_frame): Set
gtk_container_resize_mode to GTK_RESIZE_IMMEDIATE for child
frames when'x-gtk-resize-child-frames' equals 'resize-mode'.
(Fx_gtk_debug): New function to toggle interactive GTK debugging
from within Emacs.
(syms_of_xfns): New symbols Qhide and Qresize_mode.
(x-gtk-resize-child-frames): New option that allows to resize
child frames on desktops like GNOME shell (with the mutter WM)
that otherwise refuse to resize them.
* src/xterm.c (x_set_offset): Don't x_sync_with_move for child
frames, it makes moving child frames virtually impossible with
the Lucid and Motif toolkits.