feat(gui): add GUI components and displays

Source/GUI/Knob.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "../Resources.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Rotary knob — a juce::Slider styled by SerumLookAndFeel. The value readout is
+// formatted by a user-provided function via getTextFromValue().
+// ===========================================================================
+class Knob : public juce::Slider
+{
+public:
+    explicit Knob (const juce::String& name,
+                   std::function<juce::String (float)> formatter = {});
+
+    void setFormatter (std::function<juce::String (float)> f) { formatter = std::move (f); }
+
+    juce::String getTextFromValue (double value) override;
+
+private:
+    std::function<juce::String (float)> formatter;
+};
+
+} // namespace serum