Enable WPF to play well with Dependency Injection

[raffaeler]

I created my own framework to use Dependency Injection in WPF. This is done using the ASP.NET infrastructure.
I can inject pages, viewmodels and many other types, but the WPF infrastructure does not allow to inject, for example, converters or markup extensions.

Markup extensions provide an IServiceProvider which we can't access elsewhere.

This issue is about a two-phase request:

• Step 1 (short-term): Ability to provide to WPF a user-provided IServiceProvider, so that I can retrieve my objects within a markup extension.
• Step 2 (long-term): Embrace the ASP.NET DI infrastructure so that we can access configurations within converters, controls, behaviors, etc.

[lindexi]

See #4026

[raffaeler]

Great work @lindexi but I believe you should add more value to your PR by submitting a test sample that shows how/when to use the factory. Otherwise it juist looks like an internal perf gain and nothing more.
Please be more verbose :-)