[API] Adds IntelliJ ui.eventhandler package

Change-Id: I4d09be253f1de2d27139006a22bbf46c2bd3fd92
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1893
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Holger Schmeisky <google@schmeisky.com>