Allow selecting an interval of the timeline

Submitted by Alex Băluț @aleb

Description

We already allow selecting clips with drag&drop on the timeline, this is different.

The standard i and o keyboard shortcuts would allow setting the start and end of the interval. This can be used for various operations, for example playing back only the interval (possibly in a loop), or removing everything in the interval and shifting right-side content to the left.

Above the ruler at the top of the timeline there is a markers bar widget (MarkersBox in pitivi/timeline/markers.py), which displays markers but is otherwise transparent. Add a marker by clicking on the bar. We could overload this widget to display the selected interval, or we can try to display the selected interval on the ruler.

In a separate commit, we can allow drag&drop on the ruler to define the interval.