QFontComboBox sometimes became too wide because of wrong fixed size

The fixed width should not be set on QFontComboBox itself, but on the
popup.

This regressed with commit bfb25c03523757d66974189e54dda97962206ef9,
and caused assistants preferences dialog to become wider than the
desktop.

Change-Id: I2059794fc12d34837cdb7dfded80df57d102a6f0
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>