Clean up PGTK cursor blanking code

Clean up PGTK cursor blanking code

* src/pgtkterm.c (XTframe_rehighlight): Rename to
`pgtk_frame_rehighlight_hook'.  All callers changed.
(x_toggle_visible_pointer): Rename to
`pgtk_toggle_visible_pointer'.  All callers changed.
(x_setup_pointer_blanking, XTtoggle_invisible_pointer): Delete
functions.
(pgtk_create_terminal): Use new names for various hooks.
(pgtk_term_init): Set up blank cursor directly.

* src/pgtkterm.h (struct pgtk_display_info): Remove
`toggle_visible_pointer'.