[fix] #626034: Properly handle Form.Dispose to not close other visible forms.

[fix] #626034: Properly handle Form.Dispose to not close other visible forms.

	* Application.cs: At the end of the message loop in RunLoop
	don't break the loop if the Form handle has been destroyed already -
	which means that we *need* to wait for WM_QUIT and handle it
	properly instead, so it doesn't end up affecting other forms.