timeline: automate waveform resizing

Now each UndoCommand won't have to handle it (unless it can introduce
specific optimizations).

app/node/block/clip/clip.cpp

@@ -59,6 +59,26 @@ NodeInput *ClipBlock::texture_input() const
   return texture_input_;
 }
 
+void ClipBlock::LengthChangedEvent(const rational &old_length, const rational &new_length, const Timeline::MovementMode &mode)
+{
+  // Positive if made longer, negative if made shorter
+  rational diff = new_length - old_length;
+
+  if (diff < rational()) {
+    if (mode == Timeline::kTrimIn) {
+      waveform().TrimIn(-diff);
+    } else {
+      waveform().TrimOut(-diff);
+    }
+  } else {
+    if (mode == Timeline::kTrimIn) {
+      waveform().PrependSilence(diff);
+    } else {
+      waveform().AppendSilence(diff);
+    }
+  }
+}
+
 void ClipBlock::InvalidateCache(const TimeRange &range, NodeInput *from, NodeInput *source)
 {
   // If signal is from texture input, transform all times from media time to sequence time