Update devel/kvirc to latest release

Previous 5.0.0 port was a beta; this is the first real KF5- and Qt5-
compatible release. The release notes say "too many changes to be listed",
since the previous stable was six years ago. Since 5.0.0-beta1, several
(unnamed) bugfixes.

While here, fix up USES=python. KVIrc is only compatible with Python 2.7.

MFH: 2019Q1