In the past, we were using `Focus.handleHelp()` to see if we're handling a
`help` command, and print the commands supported by the handler. We also used
`strcmp_P` directly to compare (parts of) our input against command supported by
the handler. This approach works, but had multiple major disadvantages: it
duplicated strings between `handleHelp` and the `strcmp_P` calls, and it relied
on fragile substring pointers to save space.
To replace all that, this patch implements a different approach. Help handling
is split between a check (`Focus.inputMatchesHelp()`) and a
reply (`Focus.printHelp()`), the latter of which takes a list of `PSTR()`
strings, rather than one single string. This allows us to reuse the same
strings for comparing against the handler's input.
The new approach no longer uses the fragile substring pointers, nor does it use
`strcmp_P` directly, but goes through a wrapper (`Focus.inputMatchesCommand()`)
instead.
These changes lead to a more readable pattern. While we do use longer strings as
a result, there is less duplication, and the new patterns also require less
code, so we end up with saving space, at least on AVR devices.
The old methods are still available and usable, but they're deprecated.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>