allow signaling a parent process that the screen was locked

[dkondor]

(partially) fixes #633

Hi!
I've recently tried out hyprlock and it works nicely with my setup! (using Wayfire mostly) However, I also noticed the issue reported in #633 and I think it is mainly caused by not being able to delay suspend until the screen is locked.

This patch adds two (related) ways to signal to a parent process when the screen has been locked:

1. Writing to an inherited file descriptor: after the screen has been locked, a single newline is written to the FD given as an argument to the -R / --ready-fd option. Similar functionality exists in swaylock.
2. Creating a child process (with fork()) and waiting until it locks the screen (-D / --daemonize option). This ensures that when the parent process exits, the screen is locked. This is equivalent to the -fork-on-lock option in waylock or the -f / --daemonize option of swaylock.

Using the second option and e.g. swayidle, it can be ensured that the screen is locked before suspend:

swayidle -w before-sleep "hyprlock -D"

The first option can be tested e.g. with the screenlocker proxy service I'm working on:
https://github.com/dkondor/wayland-screenlock-proxy/tree/hyprlock

Caveats / TODO:

• Not tested with hypridle yet (but likely would need the equivalent of the -w option in swayidle unless I'm missing something)
• This only works well if no_fade_in = true is set in the config, since it signals success immediately when getting the locked event. I looked into how the fade-in animation is managed, and seems I would need to check when bga == 1 here, but I'm unsure how to get this information to the CHyprlock class yet.
• Both options do not work well together (if both -D and -R is given, the latter is ignored); not sure if this should be an error, or the worked around to still signal the fd given to -R

[PaideiaDilemma]

I think vaxry and me probably both thought about adding this.
Swaylock does that, but I thinks it is not an ideal solutions, since you still need to script it yourself.

Check out #609
I think we could handle this in hypridle by using such an approach.
Solving two issues at once.