Fixes v0.3

1. Fixed the bug that the boss animation may not be reset after restarting the battle 2. Fixed the bug that the tip text in the difficulty selection panel may be repeated 3. Fixed the bug that the BGM might not stop after the boss fight 4. Changed the movement that players can still trigger downward movement on the lowest layer of the platform 5. The game language will be automatically selected according to the system language if there is no predefined configuration 6. Now the player need select at least one skill to start battles