shell/Windows: Avoid memory allocations and copies when querying icons

shell/Windows: Avoid memory allocations and copies when querying icons

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.