[Enhancement] Automatically importing specific resources

[mattleibow]

Summary

When creating a library, there does not appear to be a way to automagically import the required styles and templates.

API Changes

Not sure what the best way to do this as I can't think of all possible issues this may cause, however if we follow the UWP and WPF pattern, they have a special path in the library: Themes\Generic.xaml which is automatically imported. There are also a few other theme names, but they seem to be for specific styles of Windows.

See more: https://docs.microsoft.com/en-us/dotnet/framework/wpf/controls/control-authoring-overview

Intended Use Case

My particular use case is that I want to create a templated control and include a default set of styles for this new control in the library.

Current Implementations

There are currently 2 ways to get the same sort of operation now.

User

The first is on the user to import the dictionary in App.xaml:

<Application .. xmlns:themes="clr-namespace:SkiaSharp.Extended.Controls.Themes;assembly=SkiaSharp.Extended.Controls" x:Class="SkiaSharpDemo.App">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <themes:Generic />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

This is "nice" but everyone will forget to do it.

Library

The other way which is "automatic" is done via the control constructor:

public class NewControl : TemplatedView
{
	public NewControl()
	{
		var merged = Application.Current?.Resources?.MergedDictionaries;
		if (merged != null)
		{
			var found = false;
			foreach (var dic in merged)
			{
				if (dic.GetType() == typeof(Themes.Generic))
				{
					found = true;
					break;
				}
			}
			if (!found)
				merged.Add(new Themes.Generic());
		}
	}

This is good in most cases, but obviously is not the greatest thing. The framework should do this all for me.

[mattleibow]

Duplicate of #9612