x11client: Replace activityList with m_activityList

KWin does not update activity status when setting `_KDE_NET_WM_ACTIVITIES`
to nullUuid outside of KWin, which causes "All Activities" not working
as expected for KWindowSystem.

`X11Client::activityList` is not kept in sync with
`AbstractClient::m_activityList`. Move `m_activityList` from private to
protected, and use it in `X11Client::readActivities`

BUG: 440496

src/abstract_client.h

@@ -1238,6 +1238,8 @@ protected:
 
     void cleanTabBox();
 
+    QStringList m_activityList;
+
 private Q_SLOTS:
     void shadeHover();
     void shadeUnhover();
@@ -1265,7 +1267,6 @@ private:
     ShadeMode m_shadeMode = ShadeNone;
     QVector <VirtualDesktop *> m_desktops;
 
-    QStringList m_activityList;
     int m_activityUpdatesBlocked = 0;
     bool m_blockedActivityUpdatesRequireTransients = false;
 

src/x11client.cpp

@@ -2409,16 +2409,16 @@ void X11Client::readActivities(Xcb::StringProperty &property)
 
     if (prop == Activities::nullUuid()) {
         //copied from setOnAllActivities to avoid a redundant XChangeProperty.
-        if (!activityList.isEmpty()) {
-            activityList.clear();
+        if (!m_activityList.isEmpty()) {
+            m_activityList.clear();
             updateActivities(true);
         }
         return;
     }
     if (prop.isEmpty()) {
         //note: this makes it *act* like it's on all activities but doesn't set the property to 'ALL'
-        if (!activityList.isEmpty()) {
-            activityList.clear();
+        if (!m_activityList.isEmpty()) {
+            m_activityList.clear();
             updateActivities(true);
         }
         return;
@@ -2426,7 +2426,7 @@ void X11Client::readActivities(Xcb::StringProperty &property)
 
     newActivitiesList = prop.split(u',');
 
-    if (newActivitiesList == activityList)
+    if (newActivitiesList == m_activityList)
         return; //expected change, it's ok.
 
     //otherwise, somebody else changed it. we need to validate before reacting.

src/x11client.h

@@ -448,7 +448,6 @@ private:
     Xcb::Window m_client;
     Xcb::Window m_wrapper;
     Xcb::Window m_frame;
-    QStringList activityList;
     int m_activityUpdatesBlocked;
     bool m_blockedActivityUpdatesRequireTransients;
     Xcb::Window m_moveResizeGrabWindow;