* System.Windows.Forms.dll.sources: Add the new internal window

	* System.Windows.Forms.dll.sources: Add the new internal window
	managers

	* Form.cs: We now hook into an internal window manager instead of just an
	MDI subsystem, this is so we can have properly behaving tool windows.
	* MdiClient.cs: Naming change, MdiChildContext is now WindowManager
	* InternalWindowManager.cs: New internal class that acts as a
	window manager for tool windows and as a base for mdi windows.
	* MdiWindowManager.cs: New class that acts as a window manager for
	mdi windows.

svn path=/trunk/mcs/; revision=54314