- Enable `lexical-binding'.
- Remove `mode-icons--load-file-name' and add `mode-icons--directory'.
The directory is what is needed and when developing inside Emacs the
`load-file-name' variable isn't available. So now this variable
defaults to `default-directory' when being evaluated in a buffer.
- Add/improve docstrings.
- Simplify `mode-icons'. Don't specify the entire image specification,
but just enough for the rest of the code to figure out how to create
this specification. `get-icon-display' is then used to create the
image specification.
- Shorten the `set-mode-icon' function.
