Propagating StyleClass on ShellElements to Flyout templates

[PureWeen]

Description of Change

Added default style Classes. If users set any of these it will automatically just apply those styles to all of the items

		const string DefaultFlyoutItemLabelStyle = "FlyoutItemLabelStyle";
		const string DefaultFlyoutItemImageStyle = "FlyoutItemImageStyle";
		const string DefaultFlyoutItemStyle = "FlyoutItemLayoutStyle";

Fix UWP to use flyout/menu item templates

When a user sets the StyleClass on a BaseShellElement, propagate that StyleClass to the default template for the flyout items

All style classes are added to the Grid/Label/Image that make up the template so you can style anything

As customization grows (tabs, etc..) these styles can get applied to those as well. At which point we will most likely expand this to attached properties so you can specify Tab.StyleClass and FlyoutItem.StyleClass to articulate the style class for different contexts

 <Shell.Resources>
<!-- Create a style for the Label that's part of the flyout template-->
        <Style TargetType="Label"
               Class="FlyoutLabelClass">
            <Setter Property="TextColor"
                    Value="#000000" />
            <Setter Property="HeightRequest"
                    Value="100" />
        </Style>
<!-- Set the height of the flyout container -->
        <Style TargetType="Layout" ApplyToDerivedTypes="True"
               Class="FlyoutGridClass">
            <Setter Property="HeightRequest"
                    Value="120" />
        </Style>
<!-- Set the height of a different flyout container -->
        <Style TargetType="Layout" ApplyToDerivedTypes="True"
               Class="FlyoutGridClass2">
            <Setter Property="HeightRequest"
                    Value="50" />
            <Setter Property="BackgroundColor"
                    Value="Blue" />
        </Style>
    </Shell.Resources>
    <FlyoutItem Title="Sandbox" StyleClass="FlyoutLabelClass, FlyoutGridClass">
        <Tab>
            <ShellContent ContentTemplate="{DataTemplate local:MainPage}">
            </ShellContent>
        </Tab>
    </FlyoutItem>
    <FlyoutItem Title="Sandbox2" StyleClass="FlyoutLabelClass, FlyoutGridClass2">
        <Tab>
            <ShellContent ContentTemplate="{DataTemplate local:MainPage}">
            </ShellContent>
        </Tab>
    </FlyoutItem>

Issues Resolved

• fixes [Shell] No styling properties for ShellItem or MenuItem  #4509

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

Testing Procedure

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[PureWeen]

I just removed the cast because AFAICT _mergedStyle will always just be this

If that's not true then I think MergedStyle should be added to IStyleSelectable

[PureWeen]

Because the classes are defined on NavigeableElement I just moved the interface up to that class and removed from here to avoid confusion