WM: Remove ASCII members from wmEvent & GHOST_TEventKeyData

The `ascii` member was only kept for historic reason as some platforms
didn't support utf8 when it was first introduced.

Remove the `ascii` struct members since many checks used this as a
fall-back for utf8_buf not being set which isn't needed.
There are a few cases where it's convenient to access the ASCII value
of an event (or nil) so a function has been added to do that.

*Details*

- WM_event_utf8_to_ascii() has been added for the few cases an events
  ASCII value needs to be accessed, this just avoids having to do
  multi-byte character checks in-line.

- RNA Event.ascii remains, using utf8_buf[0] for single byte characters.

- GHOST_TEventKeyData.ascii has been removed.

- To avoid regressions non-ASCII Latin1 characters from GHOST are
  converted into multi-byte UTF8, when building X11 without
  XInput & X_HAVE_UTF8_STRING it seems like could still occur.

source/blender/windowmanager/WM_api.h

@@ -1530,6 +1530,7 @@ bool WM_event_is_modal_drag_exit(const struct wmEvent *event,
 bool WM_event_is_mouse_drag(const struct wmEvent *event);
 bool WM_event_is_mouse_drag_or_press(const wmEvent *event);
 int WM_event_drag_direction(const wmEvent *event);
+char WM_event_utf8_to_ascii(const struct wmEvent *event) ATTR_NONNULL(1) ATTR_WARN_UNUSED_RESULT;
 
 /**
  * Detect motion between selection (callers should only use this for selection picking),