Allow focus on the Qt render widget

By default widgets are set to the focus policy Qt::NoFocus which disallows manually focusing it. Changing the policy to allow clicking the widget to set focus to it allows for keyboard input when not rendering to a popout window. This commit also sets focus to the widget when showing it.

Fixes issue #158.

src/citra_qt/main.cpp

@@ -170,6 +170,7 @@ void GMainWindow::BootGame(std::string filename)
     render_window->GetEmuThread().start();
 
     render_window->show();
+    render_window->setFocus();
     OnStartGame();
 }