No longer force saving projects when project items change

Related to #616.

Forcing saving projects may cause corrupted states of F# project system. For example, here is an repro:

 - Add a new `fs` file via `Add New Item` menu
 - Close the solution (noticing that it doesn't prompt users to save because the changed project is already saved)
 - Reopen the solution
 - Notice that the newly added file isn't in the end of the relevant project.