Add setting for number of files indexed by the navigator

User may add `(:lt.objs.sidebar.navigate/set-file-limit n)`, with n
integer, to customize the maximum number of workspace files that will be
indexed by the navigator for each top workspace folder.
Defaults to 8000.
Merge pull request #1340 from ndr-qef/nav-index-limit

Add setting for number of files indexed by the navigator