[TabDeckEditor] Generalize visibility filter and extract it to a separate file (#6526)

* create class

* use new class in old code

cockatrice/src/interface/widgets/utility/visibility_change_listener.cpp

@@ -0,0 +1,26 @@
+#include "visibility_change_listener.h"
+
+#include <QEvent>
+#include <QWidget>
+
+VisibilityChangeListener::VisibilityChangeListener(QWidget *targetWidget)
+    : QObject(targetWidget), targetWidget(targetWidget)
+{
+    if (targetWidget) {
+        targetWidget->installEventFilter(this);
+    }
+}
+
+bool VisibilityChangeListener::eventFilter(QObject *o, QEvent *e)
+{
+    if (o == targetWidget && !e->spontaneous()) {
+        if (e->type() == QEvent::Show) {
+            emit visibilityChanged(true);
+        }
+
+        if (e->type() == QEvent::Hide) {
+            emit visibilityChanged(false);
+        }
+    }
+    return false;
+}

cockatrice/src/interface/widgets/utility/visibility_change_listener.h

@@ -0,0 +1,35 @@
+#ifndef COCKATRICE_VISIBILITY_LISTENER_H
+#define COCKATRICE_VISIBILITY_LISTENER_H
+
+#include <QObject>
+
+/**
+ * @brief This filter listens to the visibility changes of a target widget, emitting signals whenever the visibility of
+ * that widget changes.
+ */
+class VisibilityChangeListener : public QObject
+{
+    Q_OBJECT
+
+    QWidget *targetWidget;
+
+public:
+    /**
+     * Creates a new instance of this class, watching the targetWidget.
+     * This class automatically installs itself as an eventFilter to the targetWidget.
+     *
+     * @param targetWidget The widget to watch. Sets that widget as this object's parent.
+     */
+    explicit VisibilityChangeListener(QWidget *targetWidget);
+
+    bool eventFilter(QObject *o, QEvent *e) override;
+
+signals:
+    /**
+     * Emitted whenever the target widget's visibility changes
+     * @param visible The widget's new visibility
+     */
+    void visibilityChanged(bool visible);
+};
+
+#endif // COCKATRICE_VISIBILITY_LISTENER_H