feat(tray): handle Status property

On the `Passive` value of `Status` tray items would be hidden unless
`show-passive-items` is set to true.
On the `NeedsAttention` value of `Status` tray items will have a
`.needs-attention` CSS class.

resources/style.css

@@ -195,6 +195,15 @@ label:focus {
     background-color: #2980b9;
 }
 
+#tray > .passive {
+    -gtk-icon-effect: dim;
+}
+
+#tray > .needs-attention {
+    -gtk-icon-effect: highlight;
+    background-color: #eb4d4b;
+}
+
 #idle_inhibitor {
     background-color: #2d3436;
 }