Add timeline for camera animations

[slimbuck]

This PR:

• adds a timeline panel at the bottom of the main view
• support adding and remove camera poses
• support playing the animation, stepping through camera pose points and scrubbing
• remove the camera pose panel

Notes:

• the timeline needs styling updates to integrate better with SPLAT DATA panel
• animations currently always loop
• timeline edits can't undo/redo, this will be added in a followup PR.