Move main window's event handling to behaviors

ParaView and ParaView-derived applications have their own main window class that inherits from QMainWindow. The standard Qt approach to intercepting signals to the main window (e.g. "close application") is to reimplement QMainWindow's event methods (e.g. closeEvent(QCloseEvent*)). This polymorphic solution is not available to ParaView-derived applications comprised of plugins, however. To facilitate a plugin's ability to influence the behavior of ParaView's main window, this commit introduces pqMainWindowEventManager, a manager that emits signals when the main window receivesevents. These signals are connected to pqMainWindowEventBehavior, a behavior class for performing tasks originally executed in ParaViewMainWindow (with the exception of the splash screen).