Add optimization targets to .NET

[rolfbjarne]

Looks like the following targets are never called from .NET projects:

• _OptimizePngImages
• _OptimizePropertyLists
• _OptimizeLocalizationFiles

I don't remember a reason why, so this might very well just be an oversight.

We should look into this, and see if we can use them in .NET projects.

A possible workaround would be to add something like this to the project file:

<PropertyGroup>
    <BuildDependsOn>
        $(BuildDependsOn);
		_OptimizePngImages;
	</BuildDependsOn>
</BuildDependsOn>

(in this case to run the _OptimizePngImages target).

Ref: #20106.