Fix: Default seek, event.target, import await, forward scrubbing, fullscreen inview

[oliverfoster]

requires adaptlearning/adapt-contrib-core#612
fixes #304

Fix

• import() returns a promise
• android triggers timeupdate before seeking which prevents the seek blocking behaviour
• defaultSeekBackwardInterval and defaultSeekForwardInterval need to be converted to functions from strings at runtime
• onMediaElementTimeUpdate and onMediaElementSeeking do not have event.target on windows chrome use event.currentTarget instead
• inview + fullscreen locking