Handle missing indexes in Steam vdf file lists

[HebaruSan]

Problem

Steam apparently allows libraryfolders.vdf to skip indexes:

"libraryfolders"
{
	"0"
	{
		// removed data for brevity
	}
	"2"
	{
		// removed data for brevity
	}
}

But when we try to parse this as a List, an exception is thrown:

Unhandled Exception: System.InvalidOperationException: Cannot deserialize a non-array value to type "System.Collections.Generic.List`1".
   at ValveKeyValue.ObjectCopier.MakeObject[TObject](KVObject keyValueObject, IObjectReflector reflector)
   at CKAN.SteamLibrary..ctor(String libraryPath)
   at CKAN.GameInstanceManager..ctor(IUser user, IConfiguration configuration)
   at CKAN.CmdLine.MainClass.Execute(GameInstanceManager manager, CommonOptions opts, String[] args)
   at CKAN.CmdLine.MainClass.Main(String[] args)

Changes

• Now instead of parsing this file as a list, we parse it as a dictionary and take the .Values property
• Now these exceptions are caught, logged, and ignored so CKAN can continue loading
• Same but for shortcuts.vdf, just in case

Fixes #4278.