Timeline - Create new Tracks and Clips #10

To make Clapper a proper video creation tool, we need the ability to create content directly in the editor.

The first step in this is being able to create new clips. Here are the requirements:

1. Create new tracks
2. Set the track type from a dropdown list
3. Create clips on tracks, based on the track type
4. Drag clips on the timeline
5. Drag clips from one track to another, as long as the type is the same

Content creation within the clips will be covered in a separate PR, as this pertains to the entity editor. Primarily this feature should enable tracks and clips to be dynamically created on the timeline.