Summary:
It is very unclear at this point what a valid use case for this feature
would possibly be. The old documentation only mentions $(hostname) as
an example, which can be done with $HOSTNAME instead.

Note that $(...) is still supported in Exec lines of desktop files,
this does not require [$e] anyway (and actually works better without it,
otherwise the $ signs need to be doubled to obey kconfig $e escaping rules...).

Test Plan:
ctest passes; various testcases with $(...) in desktop files,
directory files, and config files, no longer execute commands.

Reviewers: mdawson, aacid, broulik, davidedmundson, kossebau, apol, sitter, security-team

Reviewed By: mdawson, davidedmundson

Subscribers: ZaWertun, rikmills, fvogt, ngraham, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22979

Cadcading should be cascading.

Inside desktop files we want to reach also data, cache and config home
to create files inside these directories.

REVIEW: 127462
CHANGELOG: Add support for get QStandardPaths locations inside desktop files.

A lot of kiosk stuff is actually in other frameworks, from the point of
view of applications, but KConfig provides the core functionality.  Make
the docs here describe KConfig's role, rather than KIO's or KXMLGui's.

REVIEW: 117486