This is a two part PR, it appears when I was tinkering with my waybar config in my ``~/.config/waybar/` directory and occasionally did

includes: [
  "modules/custom.jsonc",
  "modules/groups.jsonc"
]

and suddenly half of my waybar just disappeared. It took me a long time to trace the issue, since there was zero messages in logs, even with -l debug key.

So, as a result here is the PR:

1. I've added couple of warning statements that elements description is missing. My assumption is that unlike any other elements (e.g. "cpu"), "custom" and "group" elements has no defaults to fall into, so if there is no configuration found (it can be wrong include as in my case, or just a typo) then this element will always be hidden. And so it is legit to warn user that this may happen.

2. I've added config search for include files in the same manner as regular config search. The only difference, I do not want for existing configs to be broken, so it is slightly more complex than just call findConfigPath. In this implementation if user uses absolute paths they work as usual, and this change only kicks in if user uses relative paths (which I presume mostly case of newcomers).

I've tried it in my environment and it is working as expected.

Also, I am flexible, can split this PR in two if needed or do modifications if some parts of this set up are unacceptable.