[Core] null/empty categories go on the bottom

Xamarin.PropertyEditing/CategoryComparer.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+
+namespace Xamarin.PropertyEditing
+{
+	internal class CategoryComparer
+		: IComparer<string>
+	{
+		public static readonly CategoryComparer Instance = new CategoryComparer();
+
+		public int Compare (string x, string y)
+		{
+			int result = Comparer<string>.Default.Compare (x, y);
+			if (result != 0 && (String.IsNullOrEmpty (x) || String.IsNullOrEmpty (y)))
+				result *= -1;
+
+			return result;
+		}
+	}
+}

Xamarin.PropertyEditing/ViewModels/PanelViewModel.cs

@@ -96,7 +96,7 @@ namespace Xamarin.PropertyEditing.ViewModels
 			Dictionary<string, List<PropertyViewModel>> groupedTypeProperties = null;
 
 			this.arranged.Clear ();
-			foreach (var grouping in props.GroupBy (GetGroup).OrderBy (g => g.Key)) {
+			foreach (var grouping in props.GroupBy (GetGroup).OrderBy (g => g.Key, CategoryComparer.Instance)) {
 				HashSet<EditorViewModel> remainingItems = null;
 
 				if (ArrangeMode == PropertyArrangeMode.Category) {
@@ -126,7 +126,7 @@ namespace Xamarin.PropertyEditing.ViewModels
 
 			if (groupedTypeProperties != null) { // Insert type-grouped properties back in sorted.
 				int i = 0;
-				foreach (var kvp in groupedTypeProperties.OrderBy (kvp => kvp.Key)) {
+				foreach (var kvp in groupedTypeProperties.OrderBy (kvp => kvp.Key, CategoryComparer.Instance)) {
 					var group = new ObservableGrouping<string, EditorViewModel> (kvp.Key) {
 						new PropertyGroupViewModel (kvp.Key, kvp.Value, ObjectEditors)
 					};

Xamarin.PropertyEditing/Xamarin.PropertyEditing.csproj

@@ -43,6 +43,7 @@
     <Compile Include="AsyncWorkQueue.cs" />
     <Compile Include="BidirectionalDictionary.cs" />
     <Compile Include="BrushPropertyInfo.cs" />
+    <Compile Include="CategoryComparer.cs" />
     <Compile Include="Drawing\CommonAlignment.cs" />
     <Compile Include="Drawing\CommonBrush.cs" />
     <Compile Include="Drawing\CommonBrushMappingMode.cs" />