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.
We need some sort of scale for content, so we will do this with
an overlay for now. However, we will also want something to be
able to do selections in the future.
This still needs some iteration for correctness, but this sort
of gets the ball rolling.
This will make it easier to support installing just sysprof-cli and the
sysprofd helper daemon on systems where GTK is not feasible or necessary.
This does not, however, do that. It simply gets things broken up into
pieces.
