[Helper] Quick and dirty string replacer with optional keys block.

First (quick and dirty) version of a more advanced key replacer where
sets of text are only outputted if the contained key matches.

include/helper.h

@@ -336,5 +336,21 @@ void parse_ranges ( char *input, rofi_range_pair **list, unsigned int *length );
  * @param filter
  */
 void rofi_output_formatted_line ( const char *format, const char *string, int selected_line, const char *filter );
+
+
+/**
+ * @param string The string with elements to be replaced
+ * @param ...    Set of {key}, value that will be replaced, terminated by  a NULL
+ *
+ * Items {key} are replaced by the value if '{key}' is passed as key/value pair, otherwise removed from string.
+ * If the {key} is in between []  all the text between [] are removed if {key}
+ * is not found. Otherwise key is replaced and [ & ] removed.
+ *
+ * This allows for optional replacement, f.e.   '{ssh-client} [-t  {title}] -e
+ * "{cmd}"' the '-t {title}' is only there if {title} is set.
+ *
+ * @returns a new string with the keys replaced.
+ */
+char *helper_string_replace_if_exists ( char * string, ... );
 G_END_DECLS
 #endif // ROFI_HELPER_H