Work around QPlainTextEdit / QPainter bug on Mac

Actually, it's not so much "work around", it's just "disable our eye candy" for
tags on Mac. This is quite frustrating. When we do anything that has the
QPainter touch the widget from our GroupedLineEdit::paintEvent() path then the
QPlainTextEdit::paintEvent() doesn't render the text of the tags.

Fixes #298

Sort of.
But I guess tag text without eye candy is better than eye candy without tag text.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>