* Apply save order for save states and batteries.
The order for save state/battery:
1. StateDir / BatteryDir;
2. The path of the current loaded game;
3. XDG Base Dir fallback.

* Use XDG Base Dir fallback to save screenshots and recordings.

* Apply search order for all dirs except recording (not implemented yet) of SDL port.
The order for battery/save state/screenshot is:
1. StateDir/BatteryDir/ScreenshotDir;
2. The path of the current loaded game;
3. XDG Base Dir (or equivalent) fallback.

* Refactor code.

* Fix freeing and setting pointer to NULL of SDL port.