Fix to display toolbar icons under UWP Shell (partial fix for #11706).

[JogyBlack]

Description of Change

The ShellToolbarItemTemplate for UWP was using incorrect binding for the IconImageSource, resulting in
Error: Converter failed to convert value of type 'Xamarin.Forms.Internals.AsyncValue1<Windows.UI.Xaml.Media.ImageSource>' to type 'ImageSource'; BindingExpression: Path='IconImageSource' DataItem='Xamarin.Forms.ToolbarItem'; target element is 'Windows.UI.Xaml.Controls.Image' (Name='null'); target property is 'Source' (type 'ImageSource').
`
Also, after correcting the binding error, the icon was displayed above the text and was very small, so I moved it to the left of the text.

Issues Resolved

• partially fixes [Bug] UWP Shell - Toolbar buttons do not work correctly #11706

API Changes

None

Platforms Affected

• UWP

Behavioral/Visual Changes

Toolbar items under UWP Shell can now display both icon and text.

Before/After Screenshots

Testing Procedure

1. Create a default Xamarin.Forms app using Shell.
2. Add UWP project
3. Add toolbar items to the main page:
   <ContentPage.ToolbarItems> <ToolbarItem Text="Add" Clicked="AddItem_Clicked" IconImageSource="tab_feed.png"/> <ToolbarItem Text="Secondary" Order="Secondary" IconImageSource="tab_about.png"/> </ContentPage.ToolbarItems>
   When the UWP app is started, the toolbar should display both image and text.

PR Checklist

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

[dnfadmin]

All CLA requirements met.

[PureWeen]

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