Possibility to add widget under the trim slider

[LeGoffMael]

Because of the the last pull request #27 it can be difficult to add a widget along the trim slider.

This modification allow to add a widget under the trim slider as a child. There is also an all made widget to show the timeline of the video.

I also edited the example to fix the SafeArea for iphone > X model and add a theme to set the color automatically.

General changes

• child Widget parameter can be added to the TrimSlider widget