Fcitx 5 flatpak package will use its own flatpak data directory

[Weng Xuetian]

Hi all,
Right now, org.fcitx.Fcitx5 shares the data directory just like the
host regular fcitx5.

Per flathub lint requirement, on the next update, the data will be
moved to its own directory (without --filesystem=xdg-config/fcitx5).

Please migrate / reconfigure correspondingly.

Thanks,
Xuetian