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

src/components/controller.cpp

@@ -3,6 +3,7 @@
 #include <csignal>
 
 #include "components/bar.hpp"
+#include "components/builder.hpp"
 #include "components/config.hpp"
 #include "components/ipc.hpp"
 #include "components/logger.hpp"
@@ -447,12 +448,15 @@ void controller::process_inputdata() {
 bool controller::process_update(bool force) {
   const bar_settings& bar{m_bar->settings()};
   string contents;
-  string separator{bar.separator};
   string padding_left(bar.padding.left, ' ');
   string padding_right(bar.padding.right, ' ');
   string margin_left(bar.module_margin.left, ' ');
   string margin_right(bar.module_margin.right, ' ');
 
+  builder build{bar};
+  build.node(bar.separator);
+  string separator{build.flush()};
+
   for (const auto& block : m_blocks) {
     string block_contents;
     bool is_left = false;