ui: Made QtWindow resizeable with resize grip

* added size grip to bottom right corner of QtWindow
* use layouts on start screen
* added close button in about window to layout

bug id = 102

src/lib_gui/qt/window/QtStartScreen.h

@@ -47,8 +47,6 @@ private slots:
 	void updateButtons();
 
 private:
-	QPushButton* m_openProjectButton;
-	QPushButton* m_newProjectButton;
 	std::vector<QtRecentProjectButton*> m_recentProjectsButtons;
 };