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

src/util/sanitize_str.cpp

@@ -0,0 +1,24 @@
+#include <array>
+#include <string>
+#include <util/sanitize_str.hpp>
+#include <utility>
+
+namespace waybar::util {
+// replaces ``<>&"'`` with their encoded counterparts
+std::string sanitize_string(std::string str) {
+  // note: it's important that '&' is replaced first; therefor we *can't* use std::map
+  const std::pair<char, std::string> replacement_table[] = {
+      {'&', "&amp;"}, {'<', "&lt;"}, {'>', "&gt;"}, {'"', "&quot;"}, {'\'', "&apos;"}};
+  size_t startpoint;
+  for (size_t i = 0; i < (sizeof(replacement_table) / sizeof(replacement_table[0])); ++i) {
+    startpoint = 0;
+    std::pair pair = replacement_table[i];
+    while ((startpoint = str.find(pair.first, startpoint)) != std::string::npos) {
+      str.replace(startpoint, 1, pair.second);
+      startpoint += pair.second.length();
+    }
+  }
+
+  return str;
+}
+}  // namespace waybar::util