Merge pull request #3696 from AvaloniaUI/treeviewitem-header-stretch

Don't constrain header's ContentPresenter on TreeViewItem

src/Avalonia.Themes.Default/TreeViewItem.xaml

@@ -17,8 +17,7 @@
                             BorderThickness="{TemplateBinding BorderThickness}"
                             TemplatedControl.IsTemplateFocusTarget="True">
                         <Grid Name="PART_Header"
-                              ColumnDefinitions="16, Auto"
+                              ColumnDefinitions="16, *"
-                              HorizontalAlignment="Left"
                               Margin="{TemplateBinding Level, Mode=OneWay, Converter={StaticResource LeftMarginConverter}}" >
                             <ToggleButton Name="expander"
                                           Focusable="False"