Check duplicated bindings after translating keysym

1). See the issue #1926. For example, the second keybinding is not detected as a duplicate:
        bindcode Mod4+24 sticky toggle
        bindsym Mod4+q focus parent
2). To fix it, check duplicated bindings when translating the keysym to keycodes.

include/config_parser.h

@@ -31,6 +31,11 @@ struct ConfigResultIR {
 
 struct ConfigResultIR *parse_config(const char *input, struct context *context);
 
+/**
+ * launch nagbar to indicate errors in the configuration file.
+ */
+void start_config_error_nagbar(const char *configpath, bool has_errors);
+
 /**
  * Parses the given file by first replacing the variables, then calling
  * parse_config and launching i3-nagbar if use_nagbar is true.