Feature/interface improvements

[Roemer]

• Added a command line argument to set the port for the flask web server (-p or --port).
• Added a debounce for the api calls when updating the rectangles (to not stress the server too much)
• Added a toggle to switch between preview stream and still image

I really like this project as I do some slideshows for my family quite often and this is much nicer than the 2d ken burns effect.