Don't update the window icon with every change to the window title

Everytime the window title is changed FNA resets the app icon. This is inconsistent with XNA and a little odd. The initialization in the constructor will use the default window title for the icon which is a generally better way to go. Plus there's simply no reason to change the icon every time the window title changes. 

This might break games if they relied on this working (e.g. they had one name for the assembly, but changed the window title to the correct name that matched the image), but I'm not sure how much weight to  assign to that use case. If someone really wanted to change the icon, I think adding an extension to the GameWindow to change the icon is the better route.