BWindow: use Unicode lowercase method

src/kits/interface/Window.cpp

@@ -32,6 +32,7 @@
 #include <Roster.h>
 #include <Screen.h>
 #include <String.h>
+#include <UnicodeChar.h>
 
 #include <AppMisc.h>
 #include <AppServerLink.h>
@@ -293,8 +294,7 @@ BWindow::Shortcut::PrepareModifiers(uint32 modifiers)
 uint32
 BWindow::Shortcut::PrepareKey(uint32 key)
 {
-	return tolower(key);
-		// TODO: support unicode and/or more intelligent key mapping
+	return BUnicodeChar::ToLower(key);
 }