[ExpandableListItem] Make colored text 100% opaque

Otherwise it can be difficult to see, especially with dark backgrounds.
Also, let's set the opacity directly rather than using the enabled
property to approximate it.

BUG: 427171
FIXED-IN: 5.76

src/declarativeimports/plasmaextracomponents/qml/ExpandableListItem.qml

@@ -421,10 +421,12 @@ Item {
                     PlasmaComponents3.Label {
                         id: listItemSubtitle
 
-                        enabled: false
                         visible: text.length > 0
                         font: PlasmaCore.Theme.smallestFont
 
+                        // Otherwise colored text can be hard to see
+                        opacity: color === PlasmaCore.Theme.textColor ? 0.7 : 1.0
+
                         Layout.fillWidth: true
 
                         textFormat: listItem.allowStyledText ? Text.StyledText : Text.PlainText