Allow binding to HeaderTemplate and ContentTemplate of material:TabItem

[StepKie]

In our code, we wanted to reuse a named DataTemplate that was defined in our Styles.xaml:

<DataTemplate x:Key="TabHeaderTemplate" x:DataType="material:TabItem">
        <Label
            Padding="0,0,0,5"
            HorizontalOptions="Center"
            VerticalOptions="Center"
            LineBreakMode="MiddleTruncation"
            Text="{Binding Title}">
            <Label.Triggers>
                <DataTrigger Binding="{Binding IsSelected}" TargetType="Label" Value="True">
                    <Setter Property="TextColor" Value="{StaticResource Primary}" />
                    <Setter Property="Scale" Value="1.2" />
                </DataTrigger>
                <DataTrigger Binding="{Binding IsSelected}" TargetType="Label" Value="False">
                    <Setter Property="TextColor" Value="Gray" />
                    <Setter Property="Scale" Value="1" />
                </DataTrigger>
            </Label.Triggers>
        </Label>
    </DataTemplate>

and apply as an implicit style it as a default to all TabItem s like so:

<Style TargetType="material:TabItem">
  <Setter Property="HeaderTemplate" Value="{StaticResource TabHeaderTemplate}" />
</Style>

Currently, this is not possible, since TabItem.cs does not expose its properties DataTemplate and ContentTemplate as BindableProperties.

#662 would enable this possibility.