Implemented event processing for Nepomuk.

Now the full events will be pushed to Nepomuk.
The following event resources are created:
* For each Accessed event a new nuao:UsageEvent is created with both
nuao:start and nuao:end set to the same timestamp.
* For each Opened event a new nuao:DesktopEvent is created.
* For each Closed event the corresponding nuao:DesktopEvent which does
not have a value for nuao:end yet is searched and converted to a
nuao:UsageEvent with correct nuao:end value.
* For each Modified event the corresponding nuao:DesktopEvent which does
not have a value for nuao:end yet is searched and converted into a
nuao:ModificationEvent with correct nuao:end value. Then a new
nuao:DesktopEvent is created as if an Opened event had been issued.
Thus, a modification event always ends with saving the resource.
Anything after that is treated as another event.