change the behaviour of the SLS

[davidea72]

change the behaviour of the SLS, it was static now when you click on SLS it show the image as usual and when a new image is available it show the new image automatically. i change the transparecy too and reduce the slidecaption height

to test it, just run welle-cli and tune to a station with sls, when it's received click on it and wait for the new image

[AlbrechtL]

Thanks @davidea72 .

@mpbraendli Can you have a look, please?