[Feature request] Add a hold_for parameter to be able to customize on screen time

[LeCalicot]

From what I understand, currently the only parameters to adjust the on screen time are the duration parameter and the wait_to_start parameter (what the AfterEnd does is a bit obscure by the way). It seem that the element directly fades out after the fade in is finished.

We would benefit by having a hold_for parameter to keep the element dt seconds on screen before the fade out starts.

fade_in => hold => fade_out