Cleanup Page Up/Down code

Cleanup Page Up/Down code

Since BufferView::scroll() does not correct scrolling amount at
top/bottom anymore, remove its return value and add a new one to
BufferView::updateMetrics(bool) that returns this correction.

Rely on updateMetrics(false) to set the metrics straight after moving
the anchor y position.

Use assert_in_view = false when calling TextMetrics::editXY() because
this parameter does not work as advertised: if an inset not totally
visible, the code will not try to go inside it to look for a smaller
row that is totally visible.

Two small (hopfully useful) modifications:
- take into account the cursor X target instead of its screen position
- adapt the cursor position depending on
  lyxrc.mac_like_cursor_movement when hitting start or end of
  document.