Revert 8535cde836 (Remove apparently unnecessary m_hasExplicitFont,
2020-07-14) and make 9cd3ab5ebd (Improve wxGenericTreeCtrl colours/fonts
updating on theme change, 2020-07-14) really work as expected by using
not only m_hasExplicitFont, but also m_hasExplicit{Fg,Bg}Col in order to
ensure that we only update the attributes if they hadn't been explicitly
set.
This is necessary because the hack with reusing m_has{Fg,Bg}Col didn't
work under Mac where the colour wasn't used at all when they were false.
This still seems to be a problem as the behaviour is different from that
of the other ports, but fixing this in wxOSX doesn't seem to be simple,
so don't rely on this working and use separate variables instead.
Closes#18940.
4a5de04ece