ui: Added singleton QtContextMenu with global back and forward actions

* QtContextMenu holds default actions for back and forward
* additional actions can be passed to extend the default menu
* currently used in QtMainWindow and QtCodeArea
* fixed crash on undo when only view actions happened since project load

bug id = 108

src/lib_gui/qt/window/QtMainWindow.cpp

@@ -12,6 +12,7 @@
 
 #include "component/view/View.h"
 #include "component/view/CompositeView.h"
+#include "qt/utility/QtContextMenu.h"
 #include "qt/utility/utilityQt.h"
 #include "qt/view/QtViewWidgetWrapper.h"
 #include "qt/window/project_wizzard/QtProjectWizzard.h"
@@ -139,8 +140,6 @@ QtMainWindow::QtMainWindow()
 	setWindowIcon(QIcon((ResourcePaths::getGuiPath() + "icon/logo_1024_1024.png").c_str()));
 	setWindowFlags(Qt::Widget);
 
-//	QApplication::setOverrideCursor(Qt::ArrowCursor);
-
 	QApplication* app = dynamic_cast<QApplication*>(QCoreApplication::instance());
 	app->installEventFilter(new MouseReleaseFilter(this));
 	app->installEventFilter(new MouseWheelFilter(this));
@@ -314,6 +313,11 @@ void QtMainWindow::keyPressEvent(QKeyEvent* event)
 	}
 }
 
+void QtMainWindow::contextMenuEvent(QContextMenuEvent* event)
+{
+	QtContextMenu::getInstance()->showDefault(event, this);
+}
+
 void QtMainWindow::activateWindow()
 {
 	this->setEnabled(true);