Set x_gtk_use_window_move by default for fixing bug#25851 and bug#25943

Set x_gtk_use_window_move by default for fixing bug#25851 and bug#25943

This activates a change that was installed a few weeks ago but whose
ChangeLog was inadvertently dropped during its commit.  The proper
ChangeLog is included below as part of the present commit.

* src/gtkutil.c (xg_set_geometry): When x_gtk_use_window_move
is set avoid calling x_gtk_parse_geometry (Bug#25851).
(x_wm_set_size_hint): When x_gtk_use_window_move is set, set
PPosition, USPosition and USSize flags if requested.
* src/xterm.c (x_set_offset): With GTK when
x_gtk_use_window_move is set, leave it entirely to
gtk_window_move to position the window and skip any
post-adjustments (Bug#25851 and Bug#25943).
(x_gtk_use_window_move): New variable.