app: sequence management -- new-sequence dialog, folders, sequences in the project bin

- File > New > Sequence opens a real dialog (name, PAL/NTSC/HD presets,
  width/height/frame rate, progressive/interlaced); VideoParams gains an
  interlaced flag
- File > New > Folder creates a folder in the project root
- sequences are mounted under the root folder so they appear in the
  project explorer (including the auto-created Sequence 1 and orphans
  from older projects); right-click > Sequence Properties edits the
  parameters afterwards
- dropping footage onto an empty, sequence-less timeline auto-creates a
  sequence from the footage's first video stream

crates/oak-task/src/nodeops.rs

@@ -316,6 +316,7 @@ fn node_video_from_common(v: &CommonVideoParams) -> oak_node::value::VideoParams
 		frame_rate: Rational::new(v.frame_rate().0 as i64, v.frame_rate().1 as i64),
 		pixel_format: v.format().code(),
 		channels: v.channel_count(),
+		interlaced: false,
 	}
 }