config: add 'hide-when-typing'

When enabled, the mouse cursor is hidden when the user types in the
terminal. It is un-hidden when the user moves the mouse, or when the
window loses keyboard focus.

wayland.h

@@ -152,6 +152,7 @@ struct seat {
         struct wl_cursor_theme *theme;
         struct wl_cursor *cursor;
         int scale;
+        bool hidden;
 
         const char *xcursor;
         struct wl_callback *xcursor_callback;