feat: fix container default value issue.

demo/Ursa.Demo/Pages/TimelineDemo.axaml

@@ -80,6 +80,7 @@
                 Content="Step 5"
                 Header="第五步"
                 Mode="Separate"
+                TimeFormat="yyyy-MM-dd"
                 Type="Error" />
         </u:Timeline>
     </WrapPanel>

src/Ursa/Controls/Timeline/Timeline.cs

@@ -150,10 +150,11 @@ public class Timeline: ItemsControl
             {
                 t.Bind(TimelineItem.TimeProperty, TimeMemberBinding);
             }
-            t.SetCurrentValue(TimelineItem.TimeFormatProperty, TimeFormat);
-            t.SetCurrentValue(TimelineItem.IconTemplateProperty, IconTemplate);
-            t.SetCurrentValue(HeaderedContentControl.HeaderTemplateProperty, ItemTemplate);
-            t.SetCurrentValue(ContentControl.ContentTemplateProperty, DescriptionTemplate);
+
+            t.SetIfUnset(TimelineItem.TimeFormatProperty, TimeFormat);
+            t.SetIfUnset(TimelineItem.IconTemplateProperty, IconTemplate);
+            t.SetIfUnset(HeaderedContentControl.HeaderTemplateProperty, ItemTemplate);
+            t.SetIfUnset(ContentControl.ContentTemplateProperty, DescriptionTemplate);
         }
 
     }

src/Ursa/Controls/Timeline/TimelineItem.cs

@@ -190,4 +190,12 @@ public class TimelineItem: HeaderedContentControl
         _rootGrid.ColumnDefinitions[1].Width = new GridLength(mid??0);
         _rootGrid.ColumnDefinitions[2].Width = new GridLength(right??0);
     }
+
+    internal void SetIfUnset<T>(AvaloniaProperty<T> property, T value)
+    {
+        if (!IsSet(property))
+        {
+            SetCurrentValue(property, value);
+        }
+    }
 }