Allow focus only when in popout mode

Only allow manually setting focus to the rendering widget when in Single Window mode. Apply this behavior to when changing the mode while an app is running.

src/citra_qt/bootmanager.cpp

@@ -123,9 +123,6 @@ GRenderWindow::GRenderWindow(QWidget* parent) : QWidget(parent), emu_thread(this
     std::string window_title = Common::StringFromFormat("Citra | %s-%s", Common::g_scm_branch, Common::g_scm_desc);
     setWindowTitle(QString::fromStdString(window_title));
 
-    // Allow manually setting focus to the widget.
-    setFocusPolicy(Qt::ClickFocus);
-
     keyboard_id = KeyMap::NewDeviceId();
     ReloadSetKeymaps();