User Accounts plug

# Expected functionality:
- Create/delete accounts
- Change password

# Mockups:
- DanRabbit: http://danrabbit.deviantart.com/art/User-Accounts-Plug-198608643

# Upstream design:
https://live.gnome.org/Design/SystemSettings/UserAccounts

All possible properties (given by org.freedesktop.Accounts.User)

bool automatic_login
bool locked
bool system_account
int32 account_type //{Administrator, Standard}
int32 password_mode //{Regular, Set At Login, None}
string email
string home_directory
string icon_file
string language
string location
string real_name //no separation into first/last name
string shell

(I don't know all of their meanings and there's no documentation, e.g. location or set_at_login from password mode...)

And I think the inline toolbar should be used for the adding/removing accounts.

- Tom Beckmann