Move the completion function to the autocomplete script.
Fix some issues with cursor shape setting, respect user configuration.
Functions and vars start with two underscores, following the convention.
Other minor refactoring.
The only downside of this new technique is that it is no
longer possible to set XDG_DATA_DIRS to an empty var in kitty.conf
Since an empty XDG_DATA_DIRS breaks lots fo things, I dont think that
is a big concern.
Less intrusive (in some ways) than adding symlinks to ~/.config/fish
Exploits the fact that fish loads scripts from XDG_DATA_DIRS on startup.
Thanks to @page-down for noticing
