Added missing Ids to templates.

And allow ContentControl to not have a ContentPresenter in its control
template, as one isn't needed for TreeViewItem for example.

Perspex.Controls/ContentControl.cs

@@ -79,9 +79,13 @@ namespace Perspex.Controls
                 this.presenterSubscription = null;
             }
 
-            this.presenter = this.GetTemplateChild<ContentPresenter>("presenter");
-            this.presenterSubscription = this.presenter.ChildObservable
-                .Subscribe(x => this.logicalChild.SingleItem = x);
+            this.presenter = this.FindTemplateChild<ContentPresenter>("presenter");
+
+            if (this.presenter != null)
+            {
+                this.presenterSubscription = this.presenter.ChildObservable
+                    .Subscribe(x => this.logicalChild.SingleItem = x);
+            }
         }
     }
 }

Perspex.Themes.Default/ButtonStyle.cs

@@ -78,6 +78,7 @@ namespace Perspex.Themes.Default
                 Padding = new Thickness(3),
                 Content = new ContentPresenter
                 {
+                    Id = "presenter",
                     [~ContentPresenter.ContentProperty] = control[~Button.ContentProperty],
                     [~ContentPresenter.HorizontalAlignmentProperty] = control[~Button.HorizontalContentAlignmentProperty],
                     [~ContentPresenter.VerticalAlignmentProperty] = control[~Button.VerticalContentAlignmentProperty],

Perspex.Themes.Default/CheckBoxStyle.cs

@@ -83,6 +83,7 @@ namespace Perspex.Themes.Default
                         },
                         new ContentPresenter
                         {
+                            Id = "presenter",
                             Margin = new Thickness(4, 0, 0, 0),
                             VerticalAlignment = VerticalAlignment.Center,
                             [~ContentPresenter.ContentProperty] = control[~CheckBox.ContentProperty],

Perspex.Themes.Default/RadioButtonStyle.cs

@@ -82,6 +82,7 @@ namespace Perspex.Themes.Default
                         },
                         new ContentPresenter
                         {
+                            Id = "presenter",
                             Margin = new Thickness(4, 0, 0, 0),
                             VerticalAlignment = VerticalAlignment.Center,
                             [~ContentPresenter.ContentProperty] = control[~RadioButton.ContentProperty],

Perspex.Themes.Default/TabItemStyle.cs

@@ -41,6 +41,7 @@ namespace Perspex.Themes.Default
         {
             return new ContentPresenter
             {
+                Id = "presenter",
                 [~ContentPresenter.ContentProperty] = control[~TabItem.HeaderProperty],
             };
         }

Perspex.Themes.Default/ToggleButtonStyle.cs

@@ -93,6 +93,7 @@ namespace Perspex.Themes.Default
                 Padding = new Thickness(3),
                 Content = new ContentPresenter
                 {
+                    Id = "presenter",
                     [~ContentPresenter.ContentProperty] = control[~ToggleButton.ContentProperty],
                     [~ContentPresenter.HorizontalAlignmentProperty] = control[~ToggleButton.HorizontalContentAlignmentProperty],
                     [~ContentPresenter.VerticalAlignmentProperty] = control[~ToggleButton.VerticalContentAlignmentProperty],