Remember last opened directory when loading decks (#5418)

* remember last directory when loading deck

* move shared code into new dlg class

cockatrice/src/dialogs/dlg_load_deck.h

@@ -0,0 +1,21 @@
+
+#ifndef DLG_LOAD_DECK_H
+#define DLG_LOAD_DECK_H
+
+#include <QFileDialog>
+
+/**
+ * The file dialog for "Load Deck" operations.
+ * Handles remembering the most recently used deck loading directory.
+ */
+class DlgLoadDeck : public QFileDialog
+{
+    Q_OBJECT
+
+    void actAccepted();
+
+public:
+    explicit DlgLoadDeck(QWidget *parent = nullptr);
+};
+
+#endif // DLG_LOAD_DECK_H