ext/workspaces: introduce ignore-hidden option

Hide hidden workspaces by default, but add an option for overriding.

While at it, move button visibility handling to Workspaces and only
handle add/removal of buttons at WorkspaceManager. This makes it
easier to keep track if sorting is needed.

man/waybar-ext-workspaces.5.scd

@@ -47,6 +47,11 @@ Addressed by *ext/workspaces*
 	default: false ++
 	If set to true only active or urgent workspaces will be shown.
 
+*ignore-hidden*: ++
+	typeof: bool ++
+	default: true ++
+	If set to false hidden workspaces will be shown.
+
 # FORMAT REPLACEMENTS
 
 *{name}*: Name of workspace assigned by compositor.