Mk/Uses: always use colon for build/run suffix
Most USES use a colon for build/run(/test) suffixes. Change kde.mk,
qt.mk and pyqt.mk to do the same, and update all ports using that.
Document in CHANGES.
PR: 266034
Exp-run by: antoine
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36349