xdg-system-bell-v1 support

I'd like to see support for xdg-system-bell-v1. Before hacking something, I think it's best to align on expectations.

Two most obvious actions for a bell are:

• set associated window to urgent
• execute a command

The config syntax can looks something like:

# Activate urgent hint
bindbell urgent enable

# Disable the above
unbindbell urgent enable

# Only exec a command
bindbell exec mpv path/to/beep.ogg

# Do both
bindbell urgent enable, exec mpv path/to/beep.ogg

# For a specific application
for_window [app_id="foot"] bindbell urgent enable

This relies on the existing syntax for commands, so it's also possible to just less common stuff, like bindbell mark --add belled.