WinRT: no read-only paths in QStandardpaths::writableLocation

As FontsLocation, HomeLocation and RuntimeLocation are read-
only on WinRT WritableLocation should return empty strings
in these cases. In addition all the other options were
added to the switch statement in writableLocation.

Task-number: QTBUG-38581
Change-Id: Iab994556844e713c6fa02028a0ec824ecb5ee82b
Reviewed-by: Andrew Knight <andrew.knight@digia.com>