[G6] Allow option for disabling toolbar separators.

[prplwtf]

Is your feature request related to a problem? Please describe. This little separator just looks off and out of place.

Describe the solution you'd like I'd like the separator to be removed, or the option to remove it.

Describe alternatives you've considered Something like an about:config configuration option.

[fossfreak]

Set userChrome.decoration.panel_button_separator to false in about:config