Sanmill v4.9.4

• Enhance AI engine performance by 17%.
• Built-in Perfect Database files for the first 4 moves. (Thanks @ggevay)
• Support displaying numbers on pieces.
• Support saving board images to the gallery (Android 10 and later only).
• Support opening PGN files from the phone's file explorer (Android 10 and later only).
• Support rotating and flipping the board during the setup position.
• Tap on the move list to navigate directly to a specific step.