changing fcitx to be implemented as a wrapper in order to use plugins

The plugin system in fctix is not very nice to work with.  Plugins are distributed as separate packages, but they must be installed into the same directory path as fcitx itself.  fcitx doesn't have any sort of
option to look in a different directory path for it's plugins.

For instance, if you install plugins separately like below, there is no way to tell fcitx where to look for them:

`/nix/store/xxxxxxxx-plugin1/lib/fcitx/plugin1.so`

`/nix/store/xxxxxxxx-plugin2/lib/fcitx/plugin2.so`

The only solution is to install the plugins with fcitx itself.  This commit creates a wrapper fcitx package that makes it possible to enable or disable plugins.  A user can enable plugins for fcitx much like
enabling plugins for firefox or chrome.

There are also two default fcitx packages available, one with no plugins and one will all plugins enabled.  This should provide binary packages for users that don't want to compile fcitx (but still may want to use
plugins).

Currently the only plugin available is the anthy plugin.

This fixes the issue at #4550.  The issue was raised upstream at fcitx/fcitx#179, but the author doesn't seem to have any desire to fix this problem.