Fix some issues with text shadows:
* Don't abuse the background color for shadows - if the background is opaque it will make the shadows invisible. Use black shadows with bright text and white shadows with dark text instead.
* Center white shadows to create a halo effect.
* Make the shadow color for selected items depend on the text highlight color.
* Change the shadow offset in RTL mode, so the light appears to be coming from the upper right corner of the screen.