Change ProgressBar Value binding mode to OneWay by default.

src/Avalonia.Controls/ProgressBar.cs

@@ -1,6 +1,7 @@
 using System;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
+using Avalonia.Data;
 using Avalonia.Layout;
 using Avalonia.Media;
 
@@ -137,6 +138,7 @@ namespace Avalonia.Controls
 
         static ProgressBar()
         {
+            ValueProperty.OverrideMetadata<ProgressBar>(new DirectPropertyMetadata<double>(defaultBindingMode: BindingMode.OneWay));
             ValueProperty.Changed.AddClassHandler<ProgressBar>((x, e) => x.UpdateIndicatorWhenPropChanged(e));
             MinimumProperty.Changed.AddClassHandler<ProgressBar>((x, e) => x.UpdateIndicatorWhenPropChanged(e));
             MaximumProperty.Changed.AddClassHandler<ProgressBar>((x, e) => x.UpdateIndicatorWhenPropChanged(e));