Allow ExpandableListItem to not be expandable when it has no valid actions

Typically ExpandableListItem expands when you click on it or its
expander arrow, and shows a list of contextual actions. However there
may occasionally arise cirsumstances under which an item has no actions.
An example would be in the Disks & Devices applet when a device doesn't
match any of the Solid predicates and therefore gets no contextual
actions. In this case, we should hide the expander arrow and now let the
user try to expand it, because there would just be a little empty list
stub.

src/declarativeimports/plasmaextracomponents/qml/ExpandableListItem.qml

@@ -282,11 +282,33 @@ Item {
      */
     property bool isDefault: false
 
+    /*
+     * hasExpandableContent: bool (read-only)
+     * Whether or not this expandable list item is actually expandable. True if
+     * this item has either a custom view or else at least one enabled action.
+     * Otherwise false.
+     */
+    readonly property bool hasExpandableContent: {
+        // If there is custom content, assume it is expandable (otherwise what
+        // would be the point?)
+        if (customExpandedViewContent != actionsListComponent) {
+            return true;
+        }
+        // Filter out disabled items which won't appear, when determining if there
+        // are any valid actions
+        if (contextualActionsModel) {
+            return Array.from(contextualActionsModel).filter(item => item.enabled).length > 0;
+        }
+        return false;
+    }
     /*
      * expand()
      * Show the expanded view, growing the list item to its taller size.
      */
     function expand() {
+        if (!listItem.hasExpandableContent) {
+            return;
+        }
         expandedView.active = true
         listItem.itemExpanded(listItem)
     }
@@ -296,6 +318,9 @@ Item {
      * Hide the expanded view and collapse the list item to its shorter size.
      */
     function collapse() {
+        if (!listItem.hasExpandableContent) {
+            return;
+        }
         expandedView.active = false
         listItem.itemCollapsed(listItem)
     }
@@ -305,6 +330,9 @@ Item {
      * Expand or collapse the list item depending on its current state.
      */
     function toggleExpanded() {
+        if (!listItem.hasExpandableContent) {
+            return;
+        }
         expandedView.active ? listItem.collapse() : listItem.expand()
     }
 
@@ -318,7 +346,7 @@ Item {
 
     // Handle left clicks and taps
     TapHandler {
-        enabled: listItem.isEnabled
+        enabled: listItem.isEnabled && listItem.hasExpandableContent
 
         acceptedButtons: Qt.LeftButton
 
@@ -457,6 +485,7 @@ Item {
 
                 // Expand/collapse button
                 PlasmaComponents3.ToolButton {
+                    visible: listItem.hasExpandableContent
                     icon.name: expandedView.active? "collapse" : "expand"
 
                     onClicked: listItem.toggleExpanded()