Add craftroot/share to QStandardPaths::GenericDataLocation on MacOS

Summary:
Before patch:
$ qtpaths --paths GenericDataLocation
/Users/alex/Library/Application Support:/Users/alex/kde/share:/Library/Application Support:/Users/alex/kde/bin

After this change:
$ qtpaths --paths GenericDataLocation
/Users/alex/Library/Application Support:/Library/Application Support:/Users/alex/kde/bin

Also supported now:
$ QT_DATA_DIRS=/foo/bar qtpaths --paths GenericDataLocation
/Users/alex/Library/Application Support:/foo/bar:/Library/Application Support:/Users/alex/kde/bin

I have added this patch for upstream review as https://codereview.qt-project.org/#/c/238640/

Reviewers: vonreth, kfunk

Reviewed By: vonreth

Differential Revision: https://phabricator.kde.org/D15235