CVAT new UI: create a shortcut for the "split" function

[aschernov]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

Expected Behaviour

The shortcut to activate the "split track" function

Current Behaviour

The "split" function is on the bottom of the left panel. It's better to have a shortcut to activate it, especially when we look after the annotated track. In this case we don't need to lose concentration and switch our attention to find this button on the left panel.