wmaker: minor fixes for the size of an aperçu

wmaker: minor fixes for the size of an aperçu

There was a probable bug when reading settings, because the function used
was 'getInt' which would try to store the result in a 'char'. As it would
be probably easier for user to have the value directly in pixels, the
storage is now done in an int so there won't be problem anymore.

Changed the behaviour of the constant APERCU_BORDER, which would be assumed
to be the size of the border in pixel, but in previous code it was actually
the sum of the two border (1 on each side). All maths have been changed to
have it as a single border width.

Took opportunity to group variable assignation for titleHeight and
shortenTitle in a single place, because it is not convenient to have them
spread around (one value in the beginning and others later in the code) and
using default values prevents some checks that modern compiler can do to
help produce safer code.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>