Changing fcitx to be implemented as a wrapper.

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.