This reinstate commit the reverted fed28a9632,
now rebased on top of conflicting changes.
* guix/build/qt-utils.scm: Remove extraneous newlines.
(variables-for-wrapping): Add comments. Define a file type entry for each
variable definition, and use it to determine if we should look for directories
versus plain files.
<QTWEBENGINEPROCESS_PATH>: New environment variable.
(wrap-all-qt-programs): Remove trailing #t.
