Minor cleanups to PGTK code

Minor cleanups to PGTK code

* src/gtkutil.c (xg_set_geometry):
(xg_frame_set_char_size):
(x_wm_set_size_hint): Rename to `xg_wm_set_size_hint'.  All
callers changed.
* src/gtkutil.h: Update prototypes.
* src/pgtkfns.c (unwind_create_frame):
(Fx_create_frame):
(compute_tip_xy):
* src/pgtkterm.c (x_free_frame_resources):
(x_destroy_window):
(x_calc_absolute_position):
(x_set_offset):
(pgtk_set_window_size):
(x_display_pixel_height):
(x_display_pixel_width):
(x_set_parent_frame): Rename `x_foo' functions to `pgtk_foo'.
Get rid of some copy-pasted code from various places.
(pgtk_create_terminal): Clean up coding style.
* src/pgtkterm.h: Update prototypes.