Classic: Improve contrast between active and disabled buttons

Text on disabled buttons (such as Turn Done during turn processing) had the
same color as text on active buttons (black). Gray it out when the button is
disabled.

Part of #1517.

data/themes/gui-qt/Classic/resource.qss

@@ -106,6 +106,7 @@ QPushButton:checked, QToolButton:checked {
 }
 
 QPushButton:disabled, QToolButton:disabled {
+  color: #888E93;
   border-image: url(LittleFingerbutton-insensitive.png) 3 3 3 3;
 }