Emit toolTipMainTextChanged if it changes in response to a title change

Summary:
The fallback for toolTipMainText is showing the applet title.

When the applet title changes, it may result in a change of
toolTipMainText.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3760

src/scriptengines/qml/plasmoid/appletinterface.cpp

@@ -82,6 +82,13 @@ AppletInterface::AppletInterface(DeclarativeAppletScript *script, const QVariant
     connect(applet(), &Plasma::Applet::titleChanged,
             this, &AppletInterface::titleChanged);
 
+    connect(applet(), &Plasma::Applet::titleChanged,
+            this, [this]() {
+                if (m_toolTipMainText.isNull()) {
+                    emit toolTipMainTextChanged();
+                }
+            });
+
     connect(applet(), &Plasma::Applet::iconChanged,
             this, &AppletInterface::iconChanged);