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

src/node_panel_brush.h

@@ -15,6 +15,7 @@ struct BrushPresetListPlan;
 }
 namespace pp::panopainter {
 class LegacyBrushTextureListServices;
+class LegacyBrushPanelUi;
 class LegacyBrushPresetListServices;
 class LegacyBrushPresetServices;
 class LegacyBrushPresetPanelUi;
@@ -42,6 +43,7 @@ public:
 class NodePanelBrush : public Node
 {
     friend class pp::panopainter::LegacyBrushTextureListServices;
+    friend class pp::panopainter::LegacyBrushPanelUi;
 
     // brushes that are marked as deleted but file still exists
     std::vector<std::string> m_deleted;