Improve how to calculate the size of alphanumeric forms

Also, a scroll for too big alphanumeric forms has been added
Window's sizes are correctly calculated on AbstractNavtable

The logic for calculating the size of a new form is delegated to
AbstractNavtable. NavtableForms only persists the size and position
of a previously open form.
Fixes on the persistance of size and position of forms

The list of windows properties should be read from the persistance
file each time we are going to use it, because the the persistance
file can be written from other opened forms or subforms. So instead
of having it as a field is handled as a local variable that is
reseted always.
Open alphanumeric subforms centered