Rewrite statistics code

Rewrite statistics code

The statistics code is known to be very slow, because it relies on
DocIterator to go through the buffer.

This commit introduces a new Statistics class that encapsulates the
main code, along a virtual method Inset::updateStatistics() that
allows to fine-tune how counting is done inset by inset.

This is a faithful bug-for-bug reimplementation.

The new code appears to be 3x faster than the old one.

See bug #12929 for a discussion about statistics update woes.