Added settings window.

src/KeyDetectorButton.swift

@@ -0,0 +1,42 @@
+import AppKit
+import Carbon
+import OSLog
+
+protocol KeyDetectorButtonDelegate: AnyObject {
+    func keyWasSet(to keyCode: Int)
+}
+
+final class KeyDetectorButton: NSButton {
+    var defaultKey: Int?
+
+    weak var delegate: KeyDetectorButtonDelegate?
+
+    override var acceptsFirstResponder: Bool { true }
+
+    // This removes default bahavior from NSButton, thus allowing mouse up
+    // events.
+    override func mouseDown(with event: NSEvent) {}
+
+    override func mouseUp(with event: NSEvent) {
+        self.window?.makeFirstResponder(self)
+    }
+
+    override func keyDown(with event: NSEvent) {
+        if event.keyCode == kVK_Escape || event.keyCode == kVK_Return {
+        } else if event.keyCode == kVK_Delete {
+            if let key = defaultKey,
+               let character = keyName(virtualKeyCode: UInt16(key))
+            {
+                title = character
+            }
+        } else {
+            if let character =
+                keyName(virtualKeyCode: UInt16(event.keyCode))
+            {
+                title = character
+            }
+            delegate?.keyWasSet(to: Int(event.keyCode))
+        }
+        self.window?.makeFirstResponder(nil)
+    }
+}