Added an event listener to call ClearLanguageServiceRootCachesAndCollectAndFinalizeAllTransients when a solution is closed

This should free up memory when closing a solution.
Removed defaultLayout="*" as this causes FSI to be shown on first use of XS

This fixes bugzilla private bug:  21210

###F# Interactive Pad is open for new Xamarin users
I've witnessed two users install Xamarin for the first time, open the t-shirt
store app solution, and see the F# interactive pad opened by default. This pad
should not be open for C# solutions by default.