Qt4Project: Do not trigger updateCodeModels on folderChanged

This reverts commit 2fdb70bd.
Be smarter about what to do on folder changes. Trigger at most
one codemodel update per folder change signal. (Thus still fixing
the original problem 2fdb70bd fixed.) And only trigger one if the
actual file list changes. This could be further optimized to only
tell the code model about the new files, but that's for another
patch.

Task-number: QTCREATORBUG-9697
Change-Id: I78d134663f1455254caf812c27c048d4f0828242
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>