Move hooks adding/removing to separate function

Both `turn-on-auto-dim-other-buffers' and `turn-off-…' had the full
list of hooks that adob used.  Change those function so that the list
is now in a separate function – `adob--hooks' – which adds or removes
the hooks depending on the argument.
Remove pre command hook

The only purpose of `adob--pre-command-hook' was to record the
`current-buffer' before the command was executed.  This allowed
`adob--post-command-hook' to check whether the buffer changed.

A much simpler approach is to record buffer whenever it is
undimmed.  This gets rid of the pre command hook and the whole
logic is now in `adob--post-command-hook'.