Update devel/qtcreator to 4.2.1

This is a pure bugfix release, and takes care of various important bugs:
http://blog.qt.io/blog/2017/01/23/qt-creator-4-2-1-released/

while here

• define PLIST_SUB so that the pkg-plist does not have to be touched every time
• switch to the tar.xz tarball to save some Mb on disfiles.

Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D9304