Avoid synthesizing stretch on condensed font families

If an entire font family is condensed or stretched and we match by
family name, the default stretch factor of 100 will make the font
engine try to synthesize it back to medium stretched font.

The existing code is already made to deal with a stretch of 0 that is
no longer used. This patch reintroduces 0 stretch to indicate no
specific stretch has been requested. Specifically setting stretch to
100 on a QFont will introduce the old behavior.

[ChangeLog][QtGui][QFont] The default value of QFont::stretch() is
now 0 to indicate any default stretch is acceptable.

Task-number: QTBUG-48043
Change-Id: I574747f980fd4f9893df828818aae99a07b41623
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>