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.
This uses the graph_width to get the proper time range of the
visualizer row. Mostly, this is just complicated because we
the ticks to be an overlay rather than inline with the list
rows.
We can avoid using the inline scrollbar in favor of overlay
scrolling if we bridge the adjustment into the GtkViewport.
This sadly requires us to subclass things to get it just
right.
Because we are using ListBox, we can't quite do scrolling the
way we'd like (naturally using the scrolled window). So for now
we need to just disable hscrolling here.
However, it would be nice if we could teach the viewport to
handle this correctly so that it wires up the
GtkScrollable:hscrollbar-adjustment to our internal one.
There are lots of little things that need fixing, such as the
ticks positioning, and reusing surfaces as much as possible, but
this gets our "MVP" panning in place.
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.
