network: Update Proxy section widgets

According to the lastest mockups [1], the Proxy section is now
composed of a row with the state of the proxy, and a settings
button that leads to a dialog where one can configure the different
proxy settings.

This commit ports the current code to do that, and various changes
took place to made this happen. Namely:

 * A new ProxyMode enum was added to improve readability and
   improve the semantic of the code. No more random numbers
   are present.

 * The current widgets for editing proxy settings were repacked
   into a GtkStack (so that we keep an homogeneous sizing), and
   the GtkStack itself was moved into a new dialog. With that,
   we can just set the stack page, rather than controlling the
   visibility of all individual widgets.

 * Many unused widgets were removed.

 * The combo box was replaced by 3 radio buttons. Now, there's
   no need to deal with GtkTreeIters anymore. Another refactoring
   of the code that led to more readable and smaller code.

Overall, these changes made the code be more readable, smaller
codebase with a smaller surface for mistakes.

https://bugzilla.gnome.org/show_bug.cgi?id=785581