Animation preview

[slimbuck]

Add the ability to view a set of PLY files as an animation.

In this PR:

• animation mode is enabled if user loads a set of files that are detected to form an animation sequence
  • user can either drag in multiple files or a folder containing multiple files
• upon detection the timeline panel is shown
• then the user can:
  • scrub the timeline
  • play and stop the animation
  • advance one frame forward and backward
  • set playback speed

This is very much work in progress and meant only for viewing animation sets. Don't attempt to load animations and non-animations and edit a scene at the same time.

Current issues:

• Each frame of the PLY animation is loaded as usual. This means you can't do anything about badly oriented scenes.

[willeastcott]

So coooooool!

[willeastcott]

Ideally, please update the Wiki with a section on animations.