KCast 0.3.0 — pure QML, no C++ plugin, playlist + seek bar

- Removed the C++ plugin entirely (KCastBridge). All catt invocation
  goes through Plasma5Support's executable engine. No compiler needed
  to install KCast anymore — kpackagetool6 -i/-u on the .plasmoid is
  now a complete install.
- Playlist: search (regex), shuffle, repeat (off/all/one), folder and
  .m3u/.m3u8 import, multi-file drag & drop.
- Seek bar: -10s/+10s buttons plus a draggable position slider,
  backed by polling `catt info -j`.