utils: add sanitize_str to encode '&' etc.

gtk requires some chars (<>&"') to be encoded for them to render
properly. `sanitize_str` sanitizes raw strings that have such chars and
returns a properly encoded string

include/util/sanitize_str.hpp

@@ -0,0 +1,6 @@
+#pragma once
+#include <string>
+
+namespace waybar::util {
+std::string sanitize_string(std::string str);
+}  // namespace waybar::util