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/progressbar.hpp

@@ -9,10 +9,6 @@
 POLYBAR_NS
 
 namespace drawtypes {
-  // fwd
-  class label;
-  using label_t = shared_ptr<label>;
-
   class progressbar : public non_copyable_mixin<progressbar> {
    public:
     explicit progressbar(const bar_settings& bar, int width, string format);