editor: generalize ce_spin_output_with_default()

We may want to use this for default value strings other than
"automatic" in the future, so change the name to "automatic" and
pass the string to the generic helper that does the actual work.

Also, the return value for the "output" signal for the spinbutton
should be 'gboolean' not 'gint'.
editor: re-add ce_spin_output_with_default()

This time, it prints "default" in the spin button instead of "automatic"
when the spin holds the default value.
editor: populate connection right before saving it

Previously the only way the UI pages could update the connection's
settings with the new values from the UI was whenever any widget
changed, which required connecting all the widgets to the the
ce_page_changed() handler to re-run the validate method, which
populated the connection and validated it.

Not all widgets need to be validated, especially if the have
GtkAdjustments or are GtkToggleButtons, since these already enforce
valid values.

Re-validating the connection right before saving it allows these
widgets to not be hooked up to ce_page_changed() every time they
change, but still allows them to push their latest value to the
connection/setting right before the changes are saved.
editor: add page for DCB options
merge: add DCB configuration UI (bgo #711032) (rh #1068688)

Add DCB configuration page to nm-connection-editor.