This gets started on allowing the user to select a region.
Nothing is yet responding to the changes in selection, but
that can come later.
Ideally, we will truncate (or mark rows invisible/insensitive)
if they do not fall within the selected time region.
These signals are emitted when a visualizer row has been added
or removed from the view. This might be handy if you want to
resize a paned based on the natural size of the visualizers.
This adds a new helper widget SpVisualizerView to simplify using
visualizers from applications using libsysprof-ui-2 such as
Builder.
We can manage the view range, zoom, etc from this widget going
forward.
