Warn user when project has too many files

This is for #6962 and https://trello.com/c/fpu67Ep7/1375-brackets-8508-6962-search-issue-with-large-folder-issue.

Warn user when project has too many files, but only warn once per project session.

The case here is a little different, but I think for now it's ok to use the same treatment as the getAllFiles() case.  We'll probably need to change this a bit in the future when user-configurable filtering is introduced, though.