split the selection panel into one card per kind of selection

src/ui/selection/ClearBeltControl.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <vector>
+
+#include <QWidget>
+
+#include "BuildingId.h"
+#include "SelectionContext.h"
+
+// The "Clear stuck items" action of the card's runtime group (REQ-UI-BELT-CLEAR): it
+// removes every item from the selected belt, splitter and tunnel tiles, which is how a
+// stalled line is resolved.
+//
+// It acts on the whole selection rather than on one tile, which is why a selection of
+// belts and tunnel ends aggregates into a single card instead of a count summary
+// (REQ-UI-SELECTION-AGGREGATE), and why the count summary shows the action too when a
+// belt is among the selected buildings.
+class ClearBeltControl : public QWidget
+{
+    Q_OBJECT
+
+public:
+    ClearBeltControl(const SelectionContext& context,
+                     const std::vector<BuildingId>& ids, QWidget* parent = nullptr);
+
+private:
+    void clearSelectedTiles() const;
+
+    SelectionContext        m_context;
+    std::vector<BuildingId> m_ids;
+};