[core] Add option to autoexpand categories at platform

Fixes #474

Xamarin.PropertyEditing/TargetPlatform.cs

@@ -111,6 +111,16 @@ namespace Xamarin.PropertyEditing
 			set;
 		}
 
+		/// <summary>
+		/// Gets or sets whether all groups should be expanded automatically.
+		/// </summary>
+		/// <remarks>This takes precedence over <see cref="AutoExpandGroups"/>.</remarks>
+		public bool AutoExpandAll
+		{
+			get;
+			set;
+		}
+
 		/// <summary>
 		/// Gets or sets a list of the allowed arrange modes.
 		/// </summary>

Xamarin.PropertyEditing/ViewModels/PanelViewModel.cs

@@ -152,6 +152,7 @@ namespace Xamarin.PropertyEditing.ViewModels
 			}
 
 			ArrangeModes = modes;
+			AutoExpand = targetPlatform.AutoExpandAll;
 		}
 
 		public event EventHandler ArrangedPropertiesChanged;