Collect statefs value of namespaces/Screen/Blanked
Use Screen/Blanked to determine display state

If a snapshot has no display events logged in the journal, use the value
indicated by statefs at the moment of snapshot's creation. The display
was on or off the whole snapshot duration.
Don't plot snapshots whose display state we couldn't trace
Merge pull request #8 from xhaakon/master

Use Screen/Blanked to determine display state