New screen options: enable & disable ShowTitleBar & FullScreen

I added new options in the screen, which allow App Inventor users to enable/disable ShowTitleBar & ShowNotificationBar. These methods are in the form of the getter and setter methods in the block editor and also in the designer property list. But these won't be any change in the designer viewer if you decide to change them. Future work, make the changes reflected on the designer viewer. 
Change is fine, but it would be nice to separate this change from the rest of the issues in fullScreen and TitleBar, since they might require more tweaking.