Improve Clip Resizing

As a clip is resized, there is no feedback to the user on what is being trimmed off. We need to have the play-head follow the edge of the clip as it's dragged. We might need to update the preview window (i.e. the XML) to only include the clip that is being resized.

For example: I grab the edge of a clip, and start to resize it. As I start the drag, the preview is replaced with just this clip, and the preview is updated to reflect the position of the trim. When the drag is complete, the preview should return to it's normal mode (which shows all tracks / clips).