[Feature] Add panel separator as a widget

[mrsteve0924]

I use xfce and they have a panel plugin to add a visual separator.

I kind of hacked one together using launcher_cmd and launcher_icon in wf-shell.ini. But it's limited to where I can place it.

launcher_cmd_separator2 =
launcher_icon_separator2 = /usr/share/icons/Papirus/symbolic/apps/separator-symbolic.svg

here's how the separators look on my xfce panel

https://docs.xfce.org/xfce/xfce4-panel/separator

[ammen99]

fixed in #233, you can use separator<width in pixels> just like spacing in the config file.