Add plaintext reveal/conceal button

[PacketFiend]

Modern password entry field best practices stipulate that the user should be offered the option of displaying the entered text in plaintext, as per NIST 800-63B, section 10.1.

This PR adds a button to status.focusWidgets that accomplishes this, revealing the password if it is currently obfuscated, and obfuscating it if it is not.

[mtwebster]

Hi, this control should be part of the text entry itself - GtkEntry (which is what the PasswordEntry inherits) has a fairly simple way of adding icons to the beginning and end of it.

On this password entry, the left-hand position (Gtk.EntryIconPosition.PRIMARY) is already used by the keyboard layout selector (if more than one is installed), but you should be able to use the secondary (right-hand) for your purposes here, and it will look more consistent with this type of action elsewhere.

You can look in Cinnamon's source, as well as many other projects, for examples of this if you need them - this is straightforward enough even AI might give you a working example :)

Thanks!

[mtwebster]

Implemented as part of the entry box.

e32b611