Easier creation of LoadFromQueryableAsync extension method

[tomasherceg]

We'll need to wait until .NET Core 3.0 is released to be able to make LoadFromQueryableAsync method (as explained in #670) since ToListAsync and CountAsync are declared in EF Core assemblies and we didn't want DotVVM to depend on that.

However, to allow users making their own implementation of LoadFromQueryableAsync as an extension method, I have changed several methods on GridViewDataSet to be public.

Making an extension method for LoadFromQueryableAsync can now be done like this:

        public static async Task LoadFromQueryableAsync(this GridViewDataSet<T> dataSet, IQueryable<T> source)
        {
            source = dataSet.ApplyFilteringToQueryable(source);
            dataSet.Items = await dataSet.ApplyOptionsToQueryable(source).ToListAsync();
            dataSet.PagingOptions.TotalItemsCount = await source.CountAsync();
            dataSet.IsRefreshRequired = false;
        }

This also fixed one inconsistency we had on GridViewDataSet - all Apply*ToQueryable except for ApplySortingToQueryable were protected while ApplySortingToQueryable was already public. Now they are all public.

[exyi]

Ok, this makes sense.