Rename 'window-unfocused' to 'backdrop'

'window-unfocused' is too long and mentions "focus" which is historically
loaded with the meaning "input focus".

'backdrop' isn't generally used in GUI speak and still conveys the state the
widgets in an unfocused or background toplevel window are in.

gtk/gtkcssprovider.c

@@ -2073,7 +2073,7 @@ parse_selector_pseudo_class (GtkCssScanner  *scanner,
     { "inconsistent", 0, GTK_STATE_FLAG_INCONSISTENT },
     { "focused",      0, GTK_STATE_FLAG_FOCUSED },
     { "focus",        0, GTK_STATE_FLAG_FOCUSED },
-    { "window-unfocused", 0, GTK_STATE_FLAG_WINDOW_UNFOCUSED },
+    { "backdrop",     0, GTK_STATE_FLAG_BACKDROP },
     { NULL, }
   }, nth_child_classes[] = {
     { "first",        GTK_REGION_FIRST, 0 },