[PlasmaCore.IconItem] Regression: fix crash on source change

Summary:
When SvgSource is changed, old one is deleted. Connections are not automatically disconnected.
This change disconnects connections and fixes regression introduced in D28470.

BUG: 420801
FIXED-IN: 5.70

Test Plan:
STEPS TO REPRODUCE
1. click on mute button for a device
2. click on the desktop to collapse the applet
OBSERVED RESULT
crash in step 1 and in the rare case it doesn't crash it crash in step 2
EXPECTED RESULT
don't crash

Reviewers: #plasma, #frameworks, ngraham, davidedmundson

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D29314

src/declarativeimports/core/iconitem.cpp

@@ -53,7 +53,6 @@ protected:
         return m_iconItem->window();
     }
 
-private:
     IconItem *m_iconItem;
 };
 
@@ -196,6 +195,10 @@ public:
         }
     }
 
+    ~SvgSource() {
+        QObject::disconnect(m_iconItem, nullptr, m_svgIcon, nullptr);
+    }
+
     bool isValid() const override
     {
         return m_svgIcon;