Sanitize cursors after a buffer has been reloaded

Sanitize cursors after a buffer has been reloaded

When a buffer is reloaded, its content may remain the same, but the
memory allocation is new, so that the inset pointers in cursors are
now wrong. This requires to sanitize the cursors held by the buffer
views.

Before the biginset branch, some full metrics computation call that is
now removed probably did that as a side effect. Now we have to be more
precise.

To this effect, introduce WorkAreaManager::sanitizeCursors() and use
it in Buffer::reload().