WinRT: Avoid QFSFileEnginePrivate::map/unmap on Windows Phone

These methods are allowed to fail; the fallback is normally to read the
whole file. Return 0/false from these functions on Windows Phone.

Change-Id: I55eb541a823029315da2607a2c1470bd7d4fb16d
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
WinRT: Don't build the network-chat example

WinRT doesn't support QProcess, so the network-chat example shouldn't be
built there.

Change-Id: I7885a992d3b8baffd5530c694063140535240f07
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>