Wrap `adob--dim-all-buffers' into a lambda when adding as a hook

`focus-out-hook's are called with no arguments but since commit
[e64bdbd: Use `face-remap-add-relative' to not interfere with
`buffer-face-mode'], `adob--dim-all-buffers' function requires
one.  To make it work as a `focus-out-hook', it needs to be
wrapped inside of a lambda.

[mina86@mina86.com: reworded commit message]
Drop turn-{on,off}-* functions.

`turn-on-auto-dim-other-buffers' and `turn-off-auto-dim-other-buffers'
functions were not really useful.  For example, they did not set the
`auto-dim-other-buffers-mode' variable so they could not be used
instead of calling the minor mode.  Furthermore, since [cfab476: Move
hooks adding/removing to separate function], they were pretty short
thus separating them did not provide much benefits as far as
readability is concerned.  Because of that, the functions are now
removed, and their code put directly into the minor mode function.
Allow disabling “dimming on focus out” feature

Added `auto-dim-other-buffers-dim-on-focus-out' customize variable
which allows disabling of the “dim on focus out” feature.