Feature: allow a toggle to enable/disable vsync

Vsync should be off by default, as for most players it will be
better to play without vsync. Exception exist, mainly people who
play in fullscreen mode.

src/widgets/settings_widget.h

@@ -35,6 +35,7 @@ enum GameOptionsWidgets {
 	WID_GO_BASE_MUSIC_DESCRIPTION = WID_GO_BASE_MUSIC_TEXTFILE + TFT_END, ///< Description of selected base music set.
 	WID_GO_FONT_ZOOM_DROPDOWN,     ///< Dropdown for the font zoom level.
 	WID_GO_VIDEO_ACCEL_BUTTON,     ///< Toggle for video acceleration.
+	WID_GO_VIDEO_VSYNC_BUTTON,     ///< Toggle for video vsync.
 	WID_GO_REFRESH_RATE_DROPDOWN,  ///< Dropdown for all available refresh rates.
 };