Commit 2dbc5b10

 introduced usage of WorkingDirectoryAspect into the
qbs run configurations for Desktop targets. However, the design of that
class was not suitable for the way our working directory logic works:
There typically is a user-provided value and, as a fallback if the user
does not provide anything, a default value managed by (build system
specific) code. The WorkingDirectoryAspect class could not differentiate
between these two values and so the value stayed forever at the initial
default if the user did not override it, instead of adapting to the
location of the executable.
This patch makes the necessary changes to the WorkingDirectoryAspect
class so that it matches the actual use case.

Task-number: QTCREATORBUG-14891
Change-Id: I7555d0a9cb4b04b75c9215a988278db32eb1ca10
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>