Default config & stylesheet location change

Kilauncher will now look for its configuration in
/etc/kilauncher/kilauncher.yaml as well as /etc/kilauncher.yaml and
.kilauncher.yaml

Kilauncher's default location for stylesheet is now
/etc/kilauncher/stylesheet.css.
Skip unreadable desktop files

Skip unreadable .desktop files in a specified directory and put a
warning on stderr.

If .desktop files specified manually are unreadable, post an error to
stderr, but still attempt to create the button (since other things may
be specified).