Compute metrics when graphics is updated

Compute metrics when graphics is updated

Remove the old schedule_redraw_ mechanism that was only useful because
of our synchronous drawing code. Now that actual painting is
scheduled instead of forced, it becomes pointless.

Rename WorkArea::redraw(bool) to scheduleRedraw(bool), to show that
the drawing is not done right away.

In GuiView::updateInset, call scheduleRedraw(true), so that metrics
are correctly computed (this was the whole point of the exercise).