Make error popup opaque

Sending screenshots of error popups is an easy way to leak information.
But if the popup is transparent, that makes it so much easier.
For the sake of privacy, error popups should be opaque.

src/gui/Popups/UnexpectedErrorPopup.qml

@@ -13,6 +13,11 @@ HColumnPopup {
 
     property var errors: []  // [{type, message, traceback}]
 
+    background: Rectangle {
+        color: theme.controls.popup.opaqueBackground
+            ? theme.controls.popup.opaqueBackground
+            : theme.controls.popup.background // fallback
+    }
 
     contentWidthLimit: Math.min(window.width / 1.5, 864 * theme.uiScale)