move the logic setting GtkSpin port value to setup_for_type()

should fix #11.

setup_for_type() was forcefully setting the port value with the method default
port, potentially overwriting a value coming from a bookmark previously set by
init_values()

Instead, let's move the code setting the value to setup_for_type(), defaulting
to 0, then to the method port if we've detected a known scheme, and if we're
editing a bookmark, take the value from the bookmark.

tested in the following cases:
- creating a new connection -> the port value is set to the scheme default
- editing an existing bookmark -> the value comes from the bookmark
- changing the scheme type on an existing bookmark -> the port value is reset
  to the scheme default
2 jobs for master in 1 minute and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #11399
build

00:00:24

 
  Distcheck
passed #11400
distcheck

00:00:36