Focus on empty Title field

If the GUI reports that the Title field is empty, focus on it, for
consistency with commit 701db76d.

src/incidencewhatwhere.cpp

@@ -62,6 +62,13 @@ bool IncidenceWhatWhere::isDirty() const
     }
 }
 
+void IncidenceWhatWhere::focusInvalidField()
+{
+    if (mUi->mSummaryEdit->text().isEmpty()) {
+        mUi->mSummaryEdit->setFocus();
+    }
+}
+
 bool IncidenceWhatWhere::isValid() const
 {
     if (mUi->mSummaryEdit->text().isEmpty()) {

src/incidencewhatwhere.h

@@ -34,6 +34,7 @@ public:
     void save(const KCalendarCore::Incidence::Ptr &incidence) override;
     Q_REQUIRED_RESULT bool isDirty() const override;
     Q_REQUIRED_RESULT bool isValid() const override;
+    void focusInvalidField() override;
     virtual void validate();
 
 private: