Allow configuring of invalid password cooldown?

## I'm submitting a…

[ ] Bug

[X] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior
Currently, there is a hardcoded cooldown for invalid credentials of 3000ms, applied by i3lock (https://github.com/i3/i3lock/blob/main/i3lock.c#L321). Time-limited physical access to my host is not a realistic threat I face - either a hacker has no physical access, or they have it for an extended duration, not much in between.

My password is long and complex, I regularly make typos. As there is no ability to configure cool-down periods of an incorrect password but I can imagine scenarios where a user may want extended cooldown prior to the ability to reenter their password.
Instead of waiting 3+ seconds each typo, I'm currently thinking about reducing my password complexity but would prefer an option to reduce cooldown as other system aspects may be impacted by reduced complexity (ssh when password auth is enabled, for example).

## Expected Behavior
Ability to configure cooldown.

## Reproduction Instructions

## Environment
Output of `i3lock --version`:

i3lock version: