refactor control and sequencer rendering

src/model.rs

@@ -44,7 +44,7 @@ pub struct App {
     /// Optional modal dialog
     pub modal:        Option<Box<dyn Component>>,
     /// Currently focused section
-    pub section:      usize,
+    pub section:      AppSection,
     /// Whether the section is focused
     pub entered:      bool,
     /// Current frame
@@ -290,3 +290,31 @@ struct SelectionMut<'a> {
     pub phrase_id: Option<usize>,
     pub phrase:    Option<&'a mut Phrase>,
 }
+
+#[derive(PartialEq, Clone, Copy)]
+pub enum AppSection {
+    Grid,
+    Sequencer,
+    Chain,
+}
+impl Default for AppSection {
+    fn default () -> Self {
+        Self::Grid
+    }
+}
+impl AppSection {
+    pub fn prev (&mut self) {
+        *self = match self {
+            Self::Grid => Self::Chain,
+            Self::Sequencer => Self::Grid,
+            Self::Chain => Self::Sequencer,
+        }
+    }
+    pub fn next (&mut self) {
+        *self = match self {
+            Self::Grid => Self::Sequencer,
+            Self::Sequencer => Self::Chain,
+            Self::Chain => Self::Grid,
+        }
+    }
+}