unattended-setup-box: Allow users to peek into the password entry

Sometimes the password_entry will be pre-filled with a password that the user has stored in the keyring.

These changes allow for the user to remember their password before their pursue a express installation.

An express installation where the user doesn't know the password can lead to a system where the user is locked out and needs to rescue the password.

Once the icons make into Gtk, we should drop them from the data/icons folder.

This is consistent with the mockups available at https://gitlab.gnome.org/Teams/Design/os-mockups/blob/master/password-entry/password-entry.png

This doesn't fix #134 (closed) yet. We plan to allow for users to peek at the password from the VM's Properties dialog too.