user-accounts: fix wrong label padding

Hardcoded value is avoided and the label offset is calculated from
style context now.

https://bugzilla.gnome.org/show_bug.cgi?id=736009
user-accounts: replace fingerprint widgets by the UmEditableButton

The patch fixes also a wrong padding and replaces GtkNotebook.

https://bugzilla.gnome.org/show_bug.cgi?id=736009