Refactor 'grey-out' CSS

The CSS used to grey out the checkbox and label for
advanced IADS should be refactored so it can be reused for
disabling player checkboxes in the flight edit/create dialogs.

The solution is applied to all checkboxes/labels, so that
there's a consistent way of showing users when a
checkbox/label is disabled.

resources/stylesheets/style-dcs.css

@@ -231,6 +231,10 @@ QLabel{
     border: none;
 }
 
+QLabel:disabled {
+    color: #888888;
+}
+
 QLabel[style="base-title"]{
     font-size: 24px;
 }
@@ -383,6 +387,13 @@ QGroupBox::indicator:checked , QCheckBox::indicator:checked {
 image: url(resources/stylesheets/check.png);
 }
 
+QCheckBox:disabled {
+    color: #888888;
+}
+QCheckBox::indicator:disabled {
+    background-color: rgba(255, 255, 255, 5%);
+}
+
 
 /*QDialog*/
 QDialog{

resources/stylesheets/style.css

@@ -191,4 +191,15 @@ QWidget[style="baseMenuHeader"]{
 
 QLabel[style="small"]{
     font-size: 8px;
+}
+
+QCheckBox:disabled {
+    color: #888888;
+}
+QCheckBox::indicator:disabled {
+    background-color: rgba(255, 255, 255, 5%);
+}
+
+QLabel:disabled {
+    color: #888888;
 }