Mark QT4 ports/functionality for removal on 2019-03-15
While here, chase some KDE4 ports and functionality, these are scheduled for
removal on 2018-12-31. Change the default option/flavor to QT5 where applicable
or use alternative toolkits like GTK.
Submitted by: tcberner
Reviewed by: adridg, jhale, rene, tcberner
Approved by: portmgr (implicit, flavor hook)
Differential Revision: https://reviews.freebsd.org/D17741