Stems for Breves

Attaching stems to brevis (and squared brevis ) to draw longa (and squared longa):

Stems were not properly attached to brevis. This derived from several factors:

* mscore-20.xml were not precise for brevis and squared brevis glyphs
* glyphs themselves were displaced horizontally (probably to attempt some centring)
* recent changes to centre note heads in segments assume specific glyph positioning
* stem position did not take centring into account

All these factors have been made consistent among themselves.

This commit only fixes Emmentaler font; fix for Gonville will follow once this change will be tested enough.