IsMemberOf is called for every file (in the ownCloud directory or not) and
with every instance of OCOverlay (we have 5) when displaying a list of
files in Explorer.
Refactor the code to avoid copying the list of watched directories, as
well as creating a wstring from a PWWSTR for files outside watched
directories.
Also change some calls of begin_with to use isDescendantOf since it
properly handles parent paths not ending with a backslash, which could
lead to SocketAPI queries for sibling folders with a name that starts with
a watched folder name.
