Improved C++ item & templates

[JaiganeshKumaran]

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Unfortunately, while C# item & project templates were cleaned up in 1.3.0, C++ item & project templates weren't.

Changes made in this pull request:

• Removed unnecessary MyProperty property and myButton_Click handler (except for MainWindow in the project template).
• Removed usage of InitializeComponent as it's no longer recommended by C++/WinRT (see https://github.com/microsoft/cppwinrt/tree/master/nuget#initializecomponent for more details).
• Removed unnecessary include of g.cpp file. Almost every app-level XAML type must have a public constructor, which means the g.cpp file will get generated.
• Removed Microsoft copyright notice.
• Removed #include XAML headers in a template control header. This was in the C++/WinRT templates because a runtime component will not include XAML headers in pch.h, which is no longer the case with a WinUI 3 runtime component.
• Replaced boxing a hardcoded type name string with the winrt::xaml_typename function. The previous one will actually crash at runtime because it isn't a TypeName.
• Included Windows.UI.Xaml.Interop.h in pch.h because its TypeName struct is still used in WinUI 3 and is required to get the winrt::xaml_typename function.
• Made the WinUI 3 runtime component "desktop-only" as WinUI 3 cannot be used by UWP apps. This is a fix for #1780.
• Removed App.idl because it's empty and not needed.
• Indented App.xaml.cpp's content inside the implementation namespace, rather than using namespace for consistency with the rest of the code. Also removed using namespace for namespaces that aren't actually used (Controls and Navigation namespaces were needed for Frame navigation, which isn't performed in the WinUI 3 template. IInspectable is available as an alias in every WinRT implementation type, thus doesn't require prefixing it with Windows::Foundation).

[DrusTheAxe]

@Scottj1s @bpulliam

[JaiganeshKumaran]

There are a few other things that could be done. Currently, the default templates enable dot prefix for C++/WinRT generated files in a sub-namespace. For example, if you have a runtime class under the View namespace, the generated file you'll need to include will be Views.RuntimeClass.g.h - however due to a long-standing XAML Compiler bug (https://github.com/microsoft/microsoft-ui-xaml/issues/7652), it will not compile since the file generated by the XAML Compiler always seems to be under a subfolder.

Another thing is if you add one of these to a project whose root namespace itself is nested, you'll see the templates emit dots in the C++ header and source instead of double colons. I would also really appreciate being able to create an item in a subfolder and it's automatically put into a sub-namespace.

Lastly, I see that the WinUI 3 item templates show up even in a WinUI 2 project. Should we move them all from neutral to desktop?

[sotanakamura]

Could you add filters to organize files shown in Solution Explorer? The C++ project template has many more files than C#.

[evelynwu-msft]

@JaiganeshKumaran thank you so much for taking the time to make these improvements to our project templates. We reviewed this PR internally and we like it a lot although there are some things that we believe merit further discussion. Unfortunately, because of internal scheduling constraints we don't have sufficient time to have that additional discussion and include your work here in the WinAppSDK 1.4 release and resolve merge conflicts with other in-flight work that we know will occur. So here's the plan we devised to fast-track the uncontroversial parts of your PR:

1. I will create a new PR into the develop branch containing the portions of this PR that we know for certain we want to accept. This will allow for expedited code flow into the release branch.
2. Once things have settled down and you have had a chance to merge main into your topic branch we will pick up this PR where we left off.
3. The portions of this PR that weren't taken into the separate PR will target WinAppSDK 1.5.

Please rest assured that we're not trying to take credit for your work. I would much prefer it if you could drive the process from beginning to end but unfortunately our schedule won't permit that and this is what we came up with as a compromise to allow most of your changes in this PR to ship in WinAppSDK 1.4.

[sylveon]

You can make the commits in your PR as authored by Jaiganesh but committed by you - GitHub will keep the credits to him.