Add icons customization in trimmer style

[LeGoffMael]

This pull request add the possibility to display icons on the left and right side of the video trimmer (by default left and right arrow).

• Size and color of the icon are customizable.
• Size and color of the circle behind the icons are also customizable.
• Size and color of the outside lines are also customizable.
• Size and color of the position line are also customizable.