PaintBrush: Allow editing palette colors by ctrl-clicking them.

Maybe the ColorDialog class could be fashioned into something generally
usable in LibGUI, but for now it lives in the PaintBrush app. :^)

Applications/PaintBrush/ColorDialog.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <LibGUI/GDialog.h>
+
+class ColorDialog final : public GDialog {
+public:
+    explicit ColorDialog(Color, CObject* parent = nullptr);
+    virtual ~ColorDialog() override;
+    virtual const char* class_name() const override { return "ColorDialog"; }
+
+    Color color() const { return m_color; }
+
+private:
+    void build();
+
+    Color m_color;
+};