Changing existing log4j logging to use slf4j

Removed org.simantics.utils.logging.{Logger,LogManager}. Anybody using
those should just switch to using slf4j. Removed their use from
org.simantics.spreadsheet.graph.

Also changed org.simantics.utils.FileUtils to use slf4j instead of
log4j.

Also added .keep to org.simantics.logback.configuration/src folder to
help it compile properly.

refs #6705

Change-Id: Ib6d78ee964e0a05aedb332167456aee4eea2a4d7
Signed-off-by: Tuukka Lehtonen <tuukka.lehtonen@semantum.fi>