textproc/fcitx5-qt: Add Qt6 support and split into qt5, qt6, common FLAVORS

[lwhsu]

Summary

I know that the USES in each flavor can be sorted more, but let's check if
this direction makes sense first.

I haven't decided yet that we should bump fcitx5-qt's PORTREVISION to make it
5.0.8_1, or rename qt5 flavor package to fcitx5-qt5 and keep 5.0.8 version.

[tcberner]

textproc/fcitx5-qt/Makefile

23

^ how about 5 for symmmetry?

29

CMAKE_OFF=ENABLE_QT4 could be extraced outside the ifs. Also, we don't have Qt4 for years now, so maybe just drop it :)

35

^ I think you should enforce >=${DISTVERSION} here, as you want to be in sync most likely

[lwhsu]

Addressing comments, split textproc/fcitx5-qt-common to slave port because self-dependency problem

[lwhsu]

textproc/fcitx5-qt/Makefile

29

?= here for slave port.

[lwhsu]

Rebase