ruler: Added storage of a timeline interval

Currently users are not able to create intervals on the timeline and
developers are not able to attach any features relating to selecting an
interval of the timeline.

Users would like to be able to select the timeline and developers for
new features would like an easy way to access that interval.

We added new keybindings to i and o for our users to set the in and
outpoints of the interval. It is stored within ruler.py in the
ScaleRuler and can be accessed by calling get_interval.

GNOME/pitivi#1842
1 job for timeline_interval in 19 minutes and 2 seconds (queued for 28 seconds)
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