Core update. Base core features are done (main app doesn't compile)

Related Work Items: #416, #422, #423, #424

FoxTube.Core/Models/Collections/InboxCollection.cs

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+using Newtonsoft.Json;
+using System.Threading.Tasks;
+using System.Net.Http;
+using Windows.UI.Xaml.Data;
+using FoxTube.Services;
+using FoxTube.Utils;
+
+namespace FoxTube.Models.Collections
+{
+	public class InboxCollection : ViewCollection<InboxItem>
+	{
+		private int _pageNumber = 0;
+		private HttpClient _httpClient = new HttpClient();
+
+		public override async Task<LoadMoreItemsResult> LoadItems()
+		{
+			// TODO: Add backend
+			HttpResponseMessage response = await _httpClient.GetAsync($"https://xfox111.net/API/FoxTube/Inbox?" +
+				$"lang={Storage.GetValue<string>(Storage.Settings.UILanguage)}&" +
+				$"currentVersion={Metrics.CurrentVersion}&" +
+				$"itemsCount={ItemsPerRequest}&" +
+				$"iteration={_pageNumber}");
+
+			if (!response.IsSuccessStatusCode || response.StatusCode == System.Net.HttpStatusCode.NoContent)
+			{
+				HasMoreItems = false;
+				return new LoadMoreItemsResult
+				{
+					Count = 0
+				};
+			}
+
+			InboxItem[] newItems = JsonConvert.DeserializeObject<InboxItem[]>(await response.Content.ReadAsStringAsync());
+			foreach (InboxItem item in newItems)
+				Items.Add(item);
+
+			_pageNumber++;
+
+			return new LoadMoreItemsResult
+			{
+				Count = (uint)newItems.Length
+			};
+		}
+	}
+}

FoxTube.Core/Models/Collections/ViewCollection.cs

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+using System.Collections.ObjectModel;
+using System.Runtime.InteropServices.WindowsRuntime;
+using System.Threading.Tasks;
+using Windows.Foundation;
+using Windows.UI.Xaml.Data;
+
+namespace FoxTube.Models.Collections
+{
+	public abstract class ViewCollection<T> : ObservableCollection<T>, ISupportIncrementalLoading
+	{
+		public int ItemsPerRequest { get; set; }
+		public bool HasMoreItems { get; protected set; } = true;
+
+		public IAsyncOperation<LoadMoreItemsResult> LoadMoreItemsAsync(uint count) =>
+			AsyncInfo.Run((c) => LoadItems());
+
+		public abstract Task<LoadMoreItemsResult> LoadItems();
+	}
+}