Don't eagerly set wxKeyEvent position fields.

Don't eagerly set wxKeyEvent position fields.

This results in a noticeable delay when using wxGTK via a remote X11
connection for every key event as a round trip to server is needed to get the
mouse pointer position every time a key is pressed or released.

Only provide the position on demand. And explain that it's actually not very
useful as it's simply the same as the current mouse position.

Closes #14361.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72207 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775