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/texteditor/snippets/snippetssettingspage.cpp

@@ -49,7 +49,6 @@
 #include <QTextStream>
 #include <QHash>
 #include <QMessageBox>
-#include <QMainWindow>
 
 namespace TextEditor {
 namespace Internal {