#166 nsDialogs: support for keyboard cues for link control

[Anders]

Requested in http://forums.winamp.com/showthread.php?s=&threadid=284958

Only tested on XP Sp2 with visual styles on (with "Hide underlined letters for keyboard navigation until I press the Alt key" on and off (Display props->Appearance->Effects))

I also added a call to WM_ERASEBKGND to fix a unrelated focus rect erase bug, not sure if that is the best way to do it, I did not want to call FillRect myself since that would require more code to support visual styles and what not.

Ref:
http://blogs.msdn.com/oldnewthing/archive/2005/05/03/414317.aspx

[Anders]