Changed return type of :ICore::mainWindow() to QWidget*

Also removed all <QMainWindow> includes which are not needed
anymore.

Change-Id: I393c9a62a5c6df95d9f35d872e1473a4f13bcdc4
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/qmljseditor/qmljscomponentfromobjectdef.cpp

@@ -45,7 +45,6 @@
 #include <QCoreApplication>
 #include <QDir>
 #include <QFileInfo>
-#include <QMainWindow>
 #include <QMessageBox>
 
 using namespace QmlJS::AST;

src/plugins/qmljseditor/qmljseditor.cpp

@@ -85,7 +85,6 @@
 #include <QComboBox>
 #include <QHeaderView>
 #include <QInputDialog>
-#include <QMainWindow>
 #include <QToolBar>
 #include <QTreeView>
 

src/plugins/qmljseditor/qmljseditoractionhandler.cpp

@@ -37,7 +37,6 @@
 
 #include <QDebug>
 #include <QAction>
-#include <QMainWindow>
 #include <QMessageBox>
 
 namespace QmlJSEditor {

src/plugins/qmljseditor/qmljseditorfactory.cpp

@@ -50,7 +50,6 @@
 #include <QSettings>
 #include <QMessageBox>
 #include <QPushButton>
-#include <QMainWindow>
 
 using namespace QmlJSEditor;
 using namespace QmlJSEditor::Internal;