IVsSolutionRestoreService4 should extend IVsSolutionRestoreService3

[zivkan]

@davkean requested that IVsSolutionRestoreService4 should extend, or re-implement all the methods from IVsSolutionRestoreService3. This avoids needing to MEF import two interfaces.

While IVsSolutionRestoreService3 and IVsSolutionRestoreService2 do not extend the lower numbered interfaces, they are used independently. Anyone using IVsSolutionRestoreService3 will not be using IVsSolutionRestoreService. However, anyone using IVsSolutionRestoreService4 must also use IVsSolutionRestoreService3, it cannot be used independently. Technically, a caller using IVsSolutionRestoreService4 could use IVsSolutionRestoreService, but IVsSolutionRestoreService3 is a superset to support additional properties, so the older interface should probably be deprecated (but is out of scope for this issue).

Using MEF is different to using VS services (GetService<T1, T2>()), where you get the implementation of the service and then cast it to the interface, so VS's COM interfaces that do not (always) extend older interfaces is a different scenario, so should not be taken as precedent.

/cc @nkolev92 @ocallesp