opacity: work on CSS style instead of SVG attribute.
set visibility in frame to be in sync with opacity/text changes. Avoids mis-display when setting visibility=visible just after value=0.
remove manual test
add unit tests
add samples. I first tried to put all Progress Indicator samples in one file, but for the sake of simplicity and readability I created a folder which contains (almost) one sample per use case. Also removed the "test_" prefix to avoid confusion with manual tests.