Fixing default font on preference window

Default font was hardcoded as 14.
What happen if you change any other preference value is that the
application would start to use fontSize=14.

This commit loads the right value in the QDoubleSpinBox

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.eu@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>