Add Grid component

[Eilon]

Syntax something like this:

    <Grid>
        <Layout>
            <RowDefinition Height="40" GridUnitType="GridUnitType.Absolute"></RowDefinition>
            <RowDefinition Height="100" GridUnitType="GridUnitType.Absolute"></RowDefinition>
            <ColumnDefinition Width="1" GridUnitType="GridUnitType.Star"></ColumnDefinition>
            <ColumnDefinition Width="2" GridUnitType="GridUnitType.Star"></ColumnDefinition>
        </Layout>
        <Contents>
            <GridCell Row="0" Column="0" ColumnSpan="2">
                <Label Text="Wide header" BackgroundColor="Color.AliceBlue" />
            </GridCell>
            <GridCell Row="1" Column="0">
                <Switch IsToggled="true" />
            </GridCell>
            <GridCell Row="1" Column="1">
                <Entry Text="Buy cat" />
            </GridCell>
        </Contents>
    </Grid>

[Eilon]

Done in ecc8602

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