Otioview display

[KarthikRIyer]

Closes #280

This PR is on top of #677 and can be merged after that.
I've implemented this by adding a display widget that displays the currently visible frame.
Only the last two commits are relevant to this PR.

Questions posed by @reinecke :

• How is missing media handled in this?
• How are clips with TimeWarp effects shown?
• How are transitions handled?
• What if the top track is a transparent PNG overlaying a graphic in the corner?

Some suggestions:

By @reinecke :

Perhaps when there are effects on clips we could do something like overlay a tint color on the playback image along with text like "Effect: Sepia". In the "special" case of LinearTimeWarp effect, it may be best to just omit the clip frames altogether and put up text saying it's not supported?

By @jminor :

One way to approach this without leading to a full video editing application is to show the display only when someone clicks on a clip. By tying the display to the inspector, we could be visually clear that you're looking at the media reference for that clip, without connecting it to the playhead position. This is a bit like the source monitor in an NLE, rather than the record monitor.

I'll make the changes as and when these questions are answered.

[ssteinbach]

This was discussed further in the latest TSC meeting, see the notes on #280

[KarthikRIyer]

Now that the idea has been refined I'm closing this PR. I'll open a new one when it's ready.