GNOME Settings allows limitless full names, which is
actually accepted by most of the stack but may break
GNOME Settings, GNOME Shell and other user-visible
applications that show the user names.
Limit the user full name entries to 255 characters,
which is the same value used by GNOME Initial Setup.
Switch from GtkBuilder to using GtkTemplate.
Rename .[ch] and .ui files to standard names.
Rename widget IDs to be more readable.
Drop widget IDs that are not used.
Move code into the .ui file that can be.
Connect signals in swapped form.