Debugger: Fix attaching to running debug server

For example when developing on Windows, cross compiling to Linux, the
result is not executable from the Windows perspective.
Reverts f2cfd3c0 which changed the code
to use ExistingCommand to get the automatic expansion of app bundles on
OS X, and do automatic expansion of app bundles also when using path
chooser of type File. Choosing an app bundle in a path chooser of type
File would previously lead to an invalid entry in the path chooser
anyhow, because the app bundle itself is not a file.

Change-Id: Ie710c47918d2b8735009e290301ed2683e354f2c
Task-number: QTCREATORBUG-14412
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>