Locator: Fix case sensitivity of file system filter

Since the path must have correct case (for case sensitive file systems),
we should only take into account if the "file name" part of the search
string is all lowercase or not.

Change-Id: I6bd49f7ac37cbdd39b995a75f1b81083c9df74d3
Task-number: QTCREATORBUG-10843
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>