arranger: fix t keybind on main (thx @magicpotatobean)

crates/app/src/model/selection.rs

@@ -53,6 +53,16 @@ impl Selection {
             _ => None
         }
     }
+    pub fn track_header (&self, track_count: usize) -> Self {
+        use Selection::*;
+        match self {
+            Mix => Track(0),
+            Scene(_) => Mix,
+            Track(t) => Track((t + 1) % track_count),
+            TrackClip { track, .. } => Track(*track),
+            _ => todo!(),
+        }
+    }
     pub fn track_next (&self, len: usize) -> Self {
         use Selection::*;
         match self {