split the selection panel into one card per kind of selection

src/ui/selection/BeltContent.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <vector>
+
+#include "BuildingId.h"
+#include "SelectionContent.h"
+#include "SelectionContentFactory.h"
+
+// The card for a belt, a tunnel entry or a tunnel exit
+// (REQ-UI-SELECTION-CONTENT). These carry no configuration and no buffers of their own;
+// their card is the clear action alone (REQ-UI-BELT-CLEAR).
+//
+// Because that action already operates on the whole selection, several of them aggregate
+// into this one card with the count in the header (REQ-UI-SELECTION-AGGREGATE) -- the
+// splitter is not among them, as its output filters are per-object.
+class BeltContent : public SelectionContent
+{
+    Q_OBJECT
+
+public:
+    BeltContent(const SelectionContext& context, const SelectionRequest& request,
+                QWidget* parent = nullptr);
+
+protected:
+    void refreshConfiguration() override;
+    void refreshRuntime() override {}
+
+private:
+    std::vector<BuildingId> m_ids;
+};