Use Global Usings and aliases to reduce #ifdef usage

[michael-hawker]

I noticed a number of ifdefs just for doc comments like so:

#if WinUI
    /// <summary>
    /// An action that will change the state of the specified <seealso cref="Microsoft.UI.Xaml.Media.Animation.Storyboard"/> when executed.
    /// </summary>
#else
    /// <summary>
    /// An action that will change the state of the specified <seealso cref="Windows.UI.Xaml.Media.Animation.Storyboard"/> when executed.
    /// </summary>
#endif

As well as many at the top of files:

#if WinUI
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Media.Animation;
#else
using Windows.UI.Xaml;
using Windows.UI.Xaml.Media.Animation;
#endif

By using global usings and aliasing, we can reduce this duplication within the project. References:

• https://learn.microsoft.com/dotnet/core/project-sdk/msbuild-props#using
• https://learn.microsoft.com/dotnet/csharp/language-reference/keywords/using-directive

By moving the general Microsoft/Windows usings to the csproj as globals, this can remove the namespaces at the top of each file making it cleaner to maintain and get to the functional code.

By using aliases, if not in the csproj at the top, it can make it easier to maintain comments for classes. See after here:

// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

using System.Diagnostics.CodeAnalysis;

// This could also be moved to alias in csproj if desired... as we don't have too many unique types for these or they are commonly used otherwise.
#if WinUI
using Storyboard = Microsoft.UI.Xaml.Media.Animation.Storyboard;
#else
using Storyboard = Windows.UI.Xaml.Media.Animation.Storyboard;
#endif

namespace Microsoft.Xaml.Interactivity
{
    /// <summary>
    /// An action that will change the state of the specified <seealso cref="Storyboard"/> when executed.
    /// </summary>
    public sealed class ControlStoryboardAction : DependencyObject, IAction
    {

[michael-hawker]

Ah, apparently I don't have at least triage permissions here, this would be a good first issue for anyone to pick up. Just noticed again in a PR:

#if WinUI
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Markup;
#else
using Windows.UI.Xaml;
using Windows.UI.Xaml.Markup;
#endif

Would make it easier to contribute new code and not worry about a bunch of pre-amble that doesn't need to be there.