Extract canvas live stroke, node canvas state, and preset panel UI

src/legacy_brush_preset_panel_ui.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "node_panel_brush.h"
+
+namespace pp::panopainter {
+
+class LegacyBrushPresetPanelUi final {
+public:
+    static void init(NodePanelBrushPreset& owner);
+    static void handle_click(NodePanelBrushPreset& owner, Node* target);
+    static void add_brush(NodePanelBrushPreset& owner, std::shared_ptr<Brush> brush);
+    static void added(NodePanelBrushPreset& owner);
+
+private:
+    static void open_menu(NodePanelBrushPreset& owner, Node* button);
+    static void show_clear_presets_confirmation(NodePanelBrushPreset& owner);
+};
+
+} // namespace pp::panopainter