Fix checkbox update when default state changes

Make sure to update checkboxes when their default value changes
and the user hasn't already changed the value themselves.

This fixes a dangerous issue with new room switching and
room settings checkboxes keeping their value from the previous room.

src/gui/Base/HCheckBox.qml

@@ -12,6 +12,8 @@ CheckBox {
     property bool defaultChecked: false
     readonly property bool changed: checked !== defaultChecked
 
+    property bool previousDefaultChecked: false  // private
+
     function reset() { checked = defaultChecked }
 
 
@@ -92,5 +94,15 @@ CheckBox {
         }
     }
 
+    onDefaultCheckedChanged: {
+        if (checked === previousDefaultChecked)
+            checked = Qt.binding(() => defaultChecked)
+
+        previousDefaultChecked = defaultChecked
+    }
+
+    // Break binding
+    Component.onCompleted: previousDefaultChecked = previousDefaultChecked
+
     Behavior on opacity { HNumberAnimation { factor: 2 } }
 }