Moving main code from TextMetrics to BufferView with following functionality

* Recompute inset positions before checking row left edge, by
painting the row insets with drawing disabled.
* Makes sure that BufferView::draw gets involved when Update flags
is just FitCursor (see BufferView::processUpdateFlags)
* Change update strategy when left edge has changed. This allows
to redraw the row when no drawing was scheduled. Something similar needs
to be done when the row left by the cursor had a non-zero left edge.