Use "bullet" character in password input fields

[steakscience]

By default, Gtk uses uses the "black circle" character in password fields (gtk.entry)

Instead, we could set a different character "bullet", which looks a lot better. This should be done in the theme GtkEntry:invisible-char https://docs.gtk.org/gtk3/property.Entry.invisible-char.html

We can use it in:

• Budgie screensaver
• Budgie styles (for password prompts)

(picture from elsewhere)

[serebit]

This seems reasonable to me.