Migrate app target to file-system-synchronized groups

[austin-smith]

Summary

Migrate the main BitDream app target from manually managed Xcode file membership to file-system-synchronized groups.

What Changed

• converted the BitDream and AppIcons app target roots to synchronized groups
• removed manual app target source and resource membership entries from project.pbxproj
• kept widget extension shared files explicitly referenced via SOURCE_ROOT
• added app-target synchronized group exceptions for Info.plist and known non-product files
• removed the now-redundant standalone BackgroundRefresh synchronized root from the app target