browser (alt_media_player): native player support + poster + flags + restricted area: - alt_media_player will now use native player when possible. This allows to use its controls and behaviour instead of native ones. - a poster can be specified when instanciated manually - video is not preloaded anymore - handle events propagation to plays nicely when used in slideshow - a "restricted area" mode can be used to let click propagation on video border, and thus catch only play/pause in the center. This is notably useful when used in the slideshow, as border can be used to show/hide slideshow controls - player can be reset, in which case the play button overlay is put back, and video is put at its beginning - once video is played at least once, a `in_use` class is added to the element, play button overlay is removed then. This fix a bug when the overlay was still appearing when using bottom play button. - VideoPlayer has been renamed to MediaPlayer