Feature request: Flatpak support

[wengxt]

Fcitx 5 engine can be distributed via Flatpak.

https://fcitx-im.org/wiki/Install_Fcitx_5#Install_Fcitx_5_from_Flatpak
https://github.com/flathub/?q=org.fcitx.Fcitx5&type=all&language=&sort=

To support flatpak, usually it need to support:

1. Non-hardcoded path to load data, since it need to be installed to a special prefix. It can be hardcoded at compile time, but I don't really suggest that, since if you want also android support, it has to be based on the XDG_* path.
2. Icon naming

• Flatpak does not allow icon to be exported by addon. -> main Fcitx flatpak package has to bundle all icon (https://github.com/fcitx/flatpak-fcitx5/blob/4dfb22440dde4f3f76a642b6b48074f76e85b605/org.fcitx.Fcitx5.yaml#L134), as you can see we also bundle some 3rd party engine icon if they request (mcbopomofo).
• DBus System tray (e.g. the one used by KDE) can only load icon from host.
• Only icon named after the flatpak package name can be exported by flatpak, that's why all icons used by fcitx are named like org.fcitx.Fcitx5.****.png .

Reference: https://github.com/fcitx/flatpak-fcitx5/