[Toolkit] IconSource support

[kazo0]

No description provided.

[kazo0]

Ideally, we would be able to have shared resources for IconElements to be able to have default icons present in the MaterialNavigationBarStyle.

One path is to implement IconSourceElement in Uno. The problem is that because of this issue we will have a blocker for PathIcon/PathIconSource being shared in a style

[kazo0]

Ex:

<Page x:Class="App1.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:local="using:App1"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
      mc:Ignorable="d">
    <Page.Resources>
        <IconSourceElement x:Key="MyIconSourceElement">
            <IconSourceElement.IconSource>
                <PathIconSource Data="PATH DATA HERE" />
            </IconSourceElement.IconSource>
        </IconSourceElement>
        <Style x:Key="MyAppBarButtonStyle"
               TargetType="AppBarButton">
            <Setter Property="Icon" Value="{StaticResource MyIconSourceElement}" />
        </Style>
    </Page.Resources>
    <Grid>
        <StackPanel>
            <AppBarButton Style="{StaticResource MyAppBarButtonStyle}" />
            <AppBarButton Style="{StaticResource MyAppBarButtonStyle}" />
            <AppBarButton Style="{StaticResource MyAppBarButtonStyle}" />
            <AppBarButton Style="{StaticResource MyAppBarButtonStyle}" />
        </StackPanel>
    </Grid>
</Page>

This causes an exception on Windows