Improve framing tool usability

* Rename "Resize" to "Resize & Framing" from discoverability
* Add ColorPreview class for displaying a solid color
* Add border color preview

rtgui/colorpreview.h

@@ -0,0 +1,53 @@
+/*
+ *  This file is part of RawTherapee.
+ *
+ *  Copyright (c) 2004-2010 Gabor Horvath <hgabor@rawtherapee.com>
+ *
+ *  RawTherapee is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  RawTherapee is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ *  2024-2024 Daniel Gao <daniel.gao.work@gmail.com>
+ */
+
+#pragma once
+
+#include <gtkmm/drawingarea.h>
+
+/**
+ * This widget displays a singular color as its contents.
+ */
+class ColorPreview : public Gtk::DrawingArea
+{
+public:
+    ColorPreview();
+
+    // Values between 0.0 and 1.0 as in
+    // Cairo::Context::set_source_rgb()
+    void setRgb(double r, double g, double b);
+
+    // Gtk::DrawingArea
+    bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr) override;
+
+    // Gtk::Widget
+    void get_preferred_height_vfunc(int& minimum_height, int& natural_height) const override;
+    void get_preferred_width_vfunc(int& minimum_width, int& natural_width) const override;
+    void get_preferred_height_for_width_vfunc(int width, int& minimum_height,
+                                              int& natural_height) const override;
+    void get_preferred_width_for_height_vfunc(int height, int & minimum_width,
+                                              int& natural_width) const override;
+
+private:
+    double color_red;
+    double color_green;
+    double color_blue;
+};