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-node/src/sequence.rs

@@ -138,6 +138,7 @@ impl SequenceBehavior {
 			frame_rate: oak_core::Rational::new(fps_num as i64, fps_den as i64),
 			pixel_format: 4, // f32
 			channels: 4,
+			interlaced: false,
 		}];
 		self.audio_params = vec![AudioParams {
 			sample_rate,