Use forward declaration when it's possible

autotests/testindividualmaildialog.cpp

@@ -8,6 +8,7 @@ SPDX-License-Identifier: LGPL-2.0-or-later
 
 #include <KGuiItem>
 
+#include <QComboBox>
 #include <QObject>
 #include <QTest>
 

src/incidenceresource.cpp

@@ -15,6 +15,7 @@
 
 #include <KDescendantsProxyModel>
 #include <KEmailAddress>
+#include <QCompleter>
 
 using namespace IncidenceEditorNG;
 

src/individualmaildialog.cpp

@@ -9,6 +9,8 @@
 #include <KGuiItem>
 #include <KLocalizedString>
 
+#include <QComboBox>
+#include <QDialogButtonBox>
 #include <QGridLayout>
 #include <QLabel>
 #include <QPushButton>

src/individualmaildialog.h

@@ -9,13 +9,11 @@
 #include <KCalendarCore/Attendee>
 #include <QDialog>
 
-#include <QComboBox>
-#include <QDialogButtonBox>
-
 class KGuiItem;
 
 class TestIndividualMailDialog;
-
+class QDialogButtonBox;
+class QComboBox;
 namespace IncidenceEditorNG
 {
 // Shows a dialog with a question and the option to select which attendee should get the mail or to open a composer for him.