Only show timeline item controls when item is selected, to keep UI minimal and simple

[pedroms]

Describe the solution you'd like
Currently, the editing controls of a specific timeline item (like zoom and speed), are always visible, which seems unnecessary.

Additionally, as we add more of these controls (like editing an annotation, audio track, etc.), it's harder to scale the UI while keeping it simple.

I suggest only showing those specific editing controls when the user selects an item in the timeline.

Additional context