Add ability to export map stitch images.

include/lib/qstringhash.h

@@ -0,0 +1,19 @@
+#ifndef QSTRINGHASH_H
+#define QSTRINGHASH_H
+
+#include <QHash>
+#include <QString>
+#include <functional>
+
+// This is a custom hash function for QString so it can be used as
+// a key in a std::hash structure. Qt 5.14 added this function, so
+// this file should only be included in earlier versions.
+namespace std {
+  template<> struct hash<QString> {
+    std::size_t operator()(const QString& s) const noexcept {
+      return static_cast<size_t>(qHash(s));
+    }
+  };
+}
+
+#endif // QSTRINGHASH_H