refactor: improve daemon / server code

[angelofallars]

The most visible change in this PR is that I changed several function / variable names to improve clarity. I've also added some blank lines in between some lines of code.

Examples:

• check_config_xdg() -> fetch_xdg_config_path()
• check_keyboard() -> check_device_is_keyboard()
• paused -> execution_is_paused

permission_check is now check_user_permissions, and it now returns a Result<T, E> as opposed to exiting the program within the function. I made it so that all calls to exit() are in fn main() so that contributors can follow the logic of the program better.

The send_command function was defined in main. I moved it outside as function definitions shouldn't go in main.

There was also improper indentation, which resulted in some unclean closing braces in main:

                    }
                }
            }
        }
        }
    }
}

Peculiarly, the second line below is inside the Some((i, Ok(event))) block yet they're of the same indentation.

            Some((i, Ok(event))) = keyboard_stream_map.next() => {
            let keyboard_state = &mut keyboard_states[i];

I found that a large part of the code was under an if let block, if let InputEventKind::Key(key) = event.kind(). I turned the if let into a simple guard clause to fix the indentation.

[Shinyzenith]

Squashed commits into 1 to keep tree clean.