Add first-class macOS build and launch support

[elnurvl]

This PR addresses several macOS-specific issues and improves build and launch experience:

• Fixes video-mode bug.
• Fixes mouse bugs.
• Fixes game file discovery issues.
• Adds native .app bundles, application metadata, and an icon.
• On macOS, it is now possible to choose a custom game directory when the game files are stored outside the app bundle.
• Adds Xcode project generation.

For more details, see the commit messages.