split the selection panel into one card per kind of selection

src/ui/selection/StorageContent.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "BufferedBuildingContent.h"
+#include "SelectionContentFactory.h"
+
+// The card for a Salvage Bay (REQ-UI-SELECTION-CONTENT): its held scrap and nothing
+// else. It has no recipe to configure and runs no production cycle
+// (REQ-BLD-SALVAGE-BAY), so it is the buffered-building card with both of those left
+// out -- its header status says whether it is holding scrap rather than whether it is
+// producing (REQ-UI-SELECTION-STATUS).
+class StorageContent : public BufferedBuildingContent
+{
+    Q_OBJECT
+
+public:
+    StorageContent(const SelectionContext& context, const SelectionRequest& request,
+                   QWidget* parent = nullptr);
+
+protected:
+    void refreshConfiguration() override;
+    CycleInfo getCycleInfo(const Building& building) const override;
+};