As part of the port to the redesigned Online Accounts panel,
the main widget to be displayed is a GtkListBox, so we can have
finer control of the UI elements and eventually be able to put
real widgets instead of using cell renderers.
This commit, then, makes the Online Account panel use a listbox
widget in the sidebar. The behavior of the panel was not changed.
Since its using a listbox now, we can drop the custom GoaPanelAccountsModel
class.
https://bugzilla.gnome.org/show_bug.cgi?id=774222
bdde40ac15