Turn Symmetry Guides visibility on and off if mirroring is enabled

Also fixed issue with "Show Guides" view menu option and having multiple projects with guides. Only think remaining is to make the Symmetry Guides look different than regular guides. Closes #133.

src/Autoload/Global.gd

@@ -42,6 +42,8 @@ var right_cursor_tool_texture := ImageTexture.new()
 
 var image_clipboard : Image
 var play_only_tags := true
+var show_x_symmetry_axis := false
+var show_y_symmetry_axis := false
 
 # Preferences
 var theme_type : int = Theme_Types.DARK