Introduced the `ideaManagedSourceSubDirs` SettingKey.

This switch provides a workaround against gen-idea listing both `src_managed/main` and its subdirectories as root source folders.

Example of failure: Play Framework generates java routes sources in `src_managed/main/controllers/routes.java`. Gen-idea lists both `src_managed/main` and `src_managed/main/controllers` as root source folders. Compilation in IntelliJ fails with the following error:

    Error:(7, 14) routes is already defined as object routes
    public class routes {
             ^