This also fixes the ".cutout-button" style being
set in "user-accounts-dialog.css", which is wrong
because this widget is located in the main panel
and not in the add user dialog.
The implementation is based on the cc-add-user-dialog.c codes, but it is
not a pure copy&paste. It has to be changed to fit the skeleton and reflect
changes from the mockup. I've also tried to simplify the code where possible.
It also fixes some memory leaks.
