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.

FYI, I have not been able to hit this case, but thought I would add this to be safe.