split the selection panel into one card per kind of selection

src/ui/selection/BufferSection.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <map>
+#include <string>
+
+#include <QWidget>
+
+struct Building;
+class QLabel;
+
+// The input and output buffer contents of one building (REQ-UI-SINGLE-SELECTION).
+//
+// Counting what is in the buffers is the same for every building type, so it happens
+// here; what a cycle consumes and produces is not, so the owning content supplies those
+// per-cycle amounts. An item with no entry in the maps is shown without a denominator.
+class BufferSection : public QWidget
+{
+    Q_OBJECT
+
+public:
+    explicit BufferSection(QWidget* parent = nullptr);
+
+    // perCycleInputs and perCycleOutputs map an item id to the amount one production
+    // cycle consumes or produces. Both may be empty, for a building that runs no cycle.
+    void setBuffers(const Building& building,
+                    const std::map<std::string, int>& perCycleInputs,
+                    const std::map<std::string, int>& perCycleOutputs);
+
+private:
+    QLabel* m_label;
+};