Add integrations panel to configure RetroAchievements, and tweaks to API

[rsubtil]

Adds a new panel to configure integrations: Additionally, there were some changes to RetroAchievements' API:

• get_game_ratings was removed: https://github.com/RetroAchievements/RAWeb/pull/2127
• Removed any processing of raw API responses, to make it consistent and up to developers to handle (closes #349)
• Automatically refetch credentials on ERR_INVALID_CRED for further API calls
• Fetch existing settings for the test_rcheevos test scene