refactor(timeline): de-Qt oaktimeline and wrap it in a pure C ABI

- de-Qt all 11 sources: markers/workarea lose QObject/signals (list
  re-sort via direct resort() call, drawing moves to the app layer),
  explicit unique_ptr ownership for marker lists and viewer's
  workarea/markers
- timeline undo command families (split/pointer/ripple/general/track)
  now hold oaknode C handles and do all graph work through the oaknode
  C ABI (no adapter layer); gaps are attached to the project graph
  before insertion, orphan handles tracked and freed
- fix the de-Qt severed Block->Track length-changed chain with a
  direct block_length_changed() call (positions were stale after
  split/trim)
- expand oaknode C API by ~20 functions needed by timeline
  (copy_in_graph, block kind/casts, connect_element, input arrays,
  tracklist family, marker/workarea borrowed outlets)
- pure C ABI in include/timeline (marker/workarea/edit, command
  factories returning OakUndoCommand), oakcommon XML get_native
  accessors
- oaknode<->oaktimeline resolve each other at runtime; standalone
  drivers link both into test binaries
- tests: oaktimeline 117, regressions oakcommon 193 / oaknode 96 /
  oakrender 42 / oakcodec 18 / oakaudio 36 all green

include/node/sequence.h

@@ -33,6 +33,18 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Sequence texture/samples input ids (ViewerOutput::k_texture_input
+ * / k_samples_input) and the track input id format
+ * (Sequence::k_track_input_format). Pinned by test.
+ */
+#define OAKNODE_SEQUENCE_TEXTURE_INPUT "tex_in"
+#define OAKNODE_SEQUENCE_SAMPLES_INPUT "samples_in"
+#define OAKNODE_SEQUENCE_TRACK_INPUT_FORMAT "track_in_%1"
+
+/* Re-declared here so sequence.h is self-contained; see node/node.h. */
+typedef struct OakNodeNode OakNodeNode;
+
 /**
  * @brief Opaque handle to a sequence (olive::Sequence).
  *
@@ -71,6 +83,12 @@ void oaknode_sequence_free(OakNodeSequence *sequence);
  * @param type One of OAKNODE_TRACK_TYPE_VIDEO / _AUDIO / _SUBTITLE.
  * @return OAKNODE_OK, OAKNODE_E_INVALID or OAKNODE_E_NOT_FOUND (bad type).
  */
+/**
+ * @brief Borrowed cast from a sequence handle to its node handle.
+ * NULL for NULL.
+ */
+OakNodeNode *oaknode_sequence_as_node(OakNodeSequence *sequence);
+
 int oaknode_sequence_get_track_list(OakNodeSequence *sequence, int type,
 									OakNodeTrackList **out);