Update insets position in cache in more cases

Update insets position in cache in more cases

This patch makes sure that, every time a ParagraphMetrics has its
position set, the inset positions for the insets held by this
paragraph are remembered too.

This is complementary to BufferView::updatePosCache, but I do not have
hard evidence that this is required other than to increase robustness.
It may help in some cases when scrolling the document (scrollbar,
cursor up/down, page up/down).