X11: Improve performance of Window::set_cursor_icon (#1116)

* X11: Fix performance issue with rapidly resetting cursor icon

* When setting cursor icon, if the new icon value is the same as the
  current value, no messages are sent the X server.

* X11: Cache cursor objects in XConnection

* Add changelog entry

src/platform_impl/linux/x11/util/mod.rs

@@ -3,6 +3,7 @@
 
 mod atom;
 mod client_msg;
+mod cursor;
 mod format;
 mod geometry;
 mod hint;