LibGUI+ThemeEditor: Split preview-widget palette-change callback

There are two different things in ThemeEditor that want to know when a
palette changes:
1. The PreviewWidget subclass, so it can update its preview.
2. The ThemeEditor itself, so we know that the palette is modified.

Using a protected virtual function for 1 means that we can do 2 without
them fighting over the same on_palette_change callback.

Userland/Applications/ThemeEditor/PreviewWidget.cpp

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2021, Antonio Di Stefano <tonio9681@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
  *
@@ -82,14 +82,16 @@ private:
 PreviewWidget::PreviewWidget(Gfx::Palette const& initial_preview_palette)
     : GUI::AbstractThemePreview(initial_preview_palette)
 {
-    on_palette_change = [&] {
-        m_gallery->set_preview_palette(preview_palette());
-        update_preview_window_locations();
-    };
     m_gallery = add<MiniWidgetGallery>();
     set_greedy_for_hits(true);
 }
 
+void PreviewWidget::palette_changed()
+{
+    m_gallery->set_preview_palette(preview_palette());
+    update_preview_window_locations();
+}
+
 void PreviewWidget::set_color_filter(OwnPtr<Gfx::ColorBlindnessFilter> color_filter)
 {
     m_color_filter = move(color_filter);

Userland/Applications/ThemeEditor/PreviewWidget.h

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2021, Antonio Di Stefano <tonio9681@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
  *
@@ -34,6 +34,7 @@ private:
     virtual void second_paint_event(GUI::PaintEvent&) override;
     virtual void resize_event(GUI::ResizeEvent&) override;
     virtual void drop_event(GUI::DropEvent&) override;
+    virtual void palette_changed() override;
 
     void paint_hightlight_window();
     void update_preview_window_locations();

Userland/Libraries/LibGUI/AbstractThemePreview.cpp

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2021, Antonio Di Stefano <tonio9681@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
  *
@@ -71,9 +71,9 @@ void AbstractThemePreview::load_theme_bitmaps()
 void AbstractThemePreview::set_preview_palette(Gfx::Palette const& palette)
 {
     m_preview_palette = palette;
-    if (on_palette_change) {
+    palette_changed();
+    if (on_palette_change)
         on_palette_change();
-    }
     load_theme_bitmaps();
     update();
 }

Userland/Libraries/LibGUI/AbstractThemePreview.h

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2021, Antonio Di Stefano <tonio9681@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
  *
@@ -45,6 +45,8 @@ protected:
         return *m_inactive_window_icon;
     }
 
+    virtual void palette_changed() {};
+
 private:
     virtual void paint_preview(GUI::PaintEvent&) = 0;