Toplevel improvement as a subviews container without frame borders.

[BDisp]

• Added a persisting multi Toplevel BackgroundWorker threading to demonstrate as requested in the issue on How to close a toplevel dialog and then change the toplevel view in the same thread(?) #985.
• For those that don't like to use the Window because has the frame border, the Toplevel was improved to provide identical funcionaties without frame border.
• All toplevels are draggable now.

With this PR in the same console it's allows 3 types of Toplevels:

1. Non persistent toplevel always recreating the Application.Top.
2. A single persistent toplevel maintained his state. (See SingleBackgroundWorker.cs scenario)
3. Multi persistent toplevels representing Mdi childes of the Mdi container, allowing multi instances of a class with the windows navigation feature between them. (See BackgroundWorkerCollection.cs scenario)

[tig]

Can you please break the parts of this PR that are distinct into separate PRs?

[BDisp]

Can you please break the parts of this PR that are distinct into separate PRs?

Done @tig. Thanks.

[tig]

My deepest apologies for taking so long on this (and others). I've been pretty busy. But I will get through all the PRs asap!