split the selection panel into one card per kind of selection

src/ui/selection/SplitterContent.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <QPoint>
+
+#include "BuildingId.h"
+#include "SelectionContent.h"
+#include "SelectionContentFactory.h"
+
+class QLabel;
+class QListWidget;
+
+// The card for a Splitter (REQ-UI-SELECTION-CONTENT): its two per-output item filters
+// (REQ-BLD-SPLITTER) plus the clear action every belt-subsystem tile has
+// (REQ-UI-BELT-CLEAR).
+//
+// The filters are configuration, so they are shown for a construction site too and set
+// through the site's own command (REQ-BLD-SITE-CONFIG); the clear action is runtime and
+// therefore is not, because a site's tile is not registered with the belt subsystem yet.
+class SplitterContent : public SelectionContent
+{
+    Q_OBJECT
+
+public:
+    SplitterContent(const SelectionContext& context, const SelectionRequest& request,
+                    QWidget* parent = nullptr);
+
+protected:
+    void refreshConfiguration() override;
+    void refreshRuntime() override {}
+
+private:
+    // Sends the checked items of both lists as the splitter's new filters. Routed to the
+    // site command or the live command depending on what the id names.
+    void applyFilters();
+    // Rebuilds both lists from the splitter's current outputs and filters. Only run when
+    // the lists are not already populated, so a per-tick refresh cannot uncheck a box
+    // the player is in the middle of clicking.
+    void populateFilters();
+
+    BuildingId   m_id;
+    bool         m_isSite;
+    QPoint       m_tile;
+    QLabel*      m_filterALabel;
+    QListWidget* m_filterAList;
+    QLabel*      m_filterBLabel;
+    QListWidget* m_filterBList;
+};