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/tests/serializer_test.rs

@@ -270,6 +270,7 @@ fn build_full_project() -> std::sync::Arc<std::sync::Mutex<oak_node::project::Pr
 					frame_rate: Rational::new(25, 1),
 					pixel_format: 4,
 					channels: 4,
+					interlaced: false,
 				}),
 				audio: None,
 				duration: Rational::new(217600, 12800),

crates/oak-node/tests/structure_test.rs

@@ -348,6 +348,7 @@ fn footage_probe() {
 				frame_rate: Rational::new(30, 1),
 				pixel_format: 4,
 				channels: 4,
+				interlaced: false,
 			}),
 			audio: None,
 			duration: Rational::new(600, 1),