kdiff3: update to 1.8.1 (Qt5 port)

Change log:

[No change log entries for 1.8.1 available.]

Version 1.8 - 2019-03-22
===========================
-Don't enable "Choose C for Everthing" on two way merge.
-cleanup code.

Version 1.7.90 - 2017-01-23
============================
-ported to QT5/KDEFramework5
-require cmake 3.1
-abandonded Konqueror specific plugin no longer applicable.
-kde4 support dropped due to extensive changes needed for QT5/KDEFramework5
  -QT5 now required because of above changes.
-bugfix: Show Indentical files setting not applied after rescan.
-Change parameter errors to use dialog on Linux as well as console output.
-Don't sort twice when sorting in reverse order.
-fix memmory leak introduced in 0.9.91.
-change version numbering.
-scrap ClearCase support
-implement Cut/Paste.

(gutteridge)