Extract brush panel UI, app startup shell, and Win32 runtime shell

src/legacy_brush_panel_ui.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include "node_panel_brush.h"
+
+namespace pp::panopainter {
+
+class LegacyBrushPanelUi final {
+public:
+    static void init(NodePanelBrush& owner);
+    static kEventResult handle_event(NodePanelBrush& owner, Event* event);
+    static void handle_click(NodePanelBrush& owner, Node* target);
+    static void added(NodePanelBrush& owner);
+};
+
+} // namespace pp::panopainter