fzf-file-widget had several mistakes:
1) It was missing zsh -c for the tmux command to run
2) the fsel function wasn't accessible from the shell we spawned in the
new tmux pane
We solved the second issue by converting the function to an executable
fzf-fsel. We also patched the bash key bindings to make use of that
executable.
While we were at it, we improved the template by setting wrksrc
correctly and cd'ing in pre_build.
