Add QT_VERSION_STR define to OpenShotVersion.h

The CMake process will pick up Qt's Qt5Core_VERSION_STRING and
stick it in a cache variable (with forced updating each run),
and a new `#define` in the header exports it to the library and
to the Python bindings so that it can be queried at runtime.

include/OpenShotVersion.h.in

@@ -42,6 +42,9 @@
 
 #define OPENSHOT_VERSION_SO @PROJECT_SO_VERSION@         /// Shared object version number. This increments any time the API and ABI changes (so old apps will no longer link)
 
+// Useful dependency versioning
+#define QT_VERSION_STR "@QT_VERSION_STR@"
+
 #include <sstream>
 
 namespace openshot
@@ -67,4 +70,4 @@ namespace openshot
 	openshot::OpenShotVersion GetVersion();
 }
 
-#endif // OPENSHOT_VERSION_H
+#endif // OPENSHOT_VERSION_H