Add support for setting cursor location using Gtk IM Context

Add support for setting cursor location using Gtk IM Context

The existing XIC implementation doesn't work when using Gtk IM Context.
Instead, `gtk_im_context_set_cursor_location` should be used
to update the cursor information for the input method.

* src/pgtkim.c (pgtk_im_set_cursor_location): New function to tell gtk
cursor location.
* src/pgtkterm.c (pgtk_draw_window_cursor): Remove invalid code and
add new code to call the function.
* src/pgtkterm.h: Add the function declaration.