wifi: More margin_start/end extermination
wifi: Use an existing icon
goa: Some spacing tweaks

This is according to the current design,
https://wiki.gnome.org/Design/OS/InitialSetup#Layout_Guidance
network: Some spacing tweaks
timezone: Some spacing tweaks
language: Some spacing tweaks
language: Don't remove nonexisting sources
language: Chain up in dispose

Things break in hard-to-track-down ways otherwise.
language: Don't go to the next page automatically

This made sense with a tree view, where selection (single click)
and activation (double click) are distinct. With the current
list view, it is much clearer to let the user click 'Next'
explicitly.
language: Only show languages

Reduce the language chooser to show only languages. We still
select a full locale, but we will soon introduce a separate
region chooser which will allow to refine the choice of
region as a second step.
Add a region chooser

Now that the language chooser only shows languages, we add
a separate step to select a region for given language. Under
the covers, both steps select a complete locale. The language
page just selects the first locale with a given locale. The
region page shows all the locales with the same language.

If there is only one region, we skip this page altogether.