bar: Make module separator a label

Some people use text modules instead of the `separator` key in the bar
section to better configure the separator (colors, fonts).
Since we disallowed the same module being used multiple times in #1534,
this will now print an error message.

This should help with this a bit.

Ref #1913

include/drawtypes/label.hpp

@@ -18,9 +18,6 @@ namespace drawtypes {
     bool zpad{false};
   };
 
-  class label;
-  using label_t = shared_ptr<label>;
-
   class label : public non_copyable_mixin<label> {
    public:
     string m_foreground{};