Add file selection screen

src/CrossPointState.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <iosfwd>
+#include <string>
+
+class CrossPointState {
+  void serialize(std::ostream& os) const;
+  static CrossPointState* deserialize(std::istream& is);
+
+ public:
+  std::string openEpubPath;
+  ~CrossPointState() = default;
+  void saveToFile() const;
+  static CrossPointState* loadFromFile();
+};