Async mod loading during loading screen

[tinyhoot]

Changes made in this pull request

• Provide a way for mods to insert themselves into the loading screen. Mods can register tasks which must complete before the game can continue loading, preventing race conditions caused by excessive asset loading when many mods are present.
• Provide a status display during the loading screen so that users know what is currently happening. Displays the current game loading stage as well as how many mod tasks have been processed, along with how many remain.
• Have task execution windows early, "medium", and late into the loading process.
• Migrate existing Nautilus systems to use the loading screen tasks. Notably SaveDataCache.

Breaking changes

• Ideally none, the existing API should persist with changes only happening under the hood.
• Mods which override SaveDataCache's sync Load or Save methods will likely break silently, as the SaveDataHandler now calls their async counterparts.
• SaveDataCaches use this new system, which means their timing is ever so slightly different, which may trip up mods that rely on extremely precise execution timings.