Implement transitions in the preview renderer

[jbilcke-hf]

Context

Currently the transitions in Clapper do not do much:

The screenplay importer will produce the corresponding segments, but instead of displaying them in the timeline nothing will happen.

We already have a dedicated ticket for the MP4 file exporter and the MLT file exporter (see #62), so what needs to be done here is to implement it in the preview renderer.

Solution

Transitions should have the desired visual outcome in the preview renderer (a.k. the video monitor).

Requirements

• At the minimum we should be able to support a simple fade and fade to/from black type of transition.
• The duration of each transition should be taken into account (even a simple black "CUT TO" transition has a duration, eg. long black cuts in some TV shows).