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-cli/src/engine.rs

@@ -255,6 +255,7 @@ pub fn set_sequence_video_params(
 				frame_rate: Rational::new(i64::from(fr_num), i64::from(fr_den)),
 				pixel_format: 4, // f32
 				channels: 4,
+				interlaced: false,
 			});
 		} else {
 			let v = &mut s.video_params[0];