Move endsWith() to UtilityClass.cpp and put it in header file

This ensures that endsWith() can be used outside of editor.cpp.

When leo60228 originally wrote endsWith(), it was static, but I asked
him on Discord just now and he more-or-less confirmed that it's fine if
it's not static. If it was static, it would be confined to
UtilityClass.cpp now instead!

desktop_version/src/UtilityClass.h

@@ -13,6 +13,8 @@ std::vector<std::string> split(const std::string &s, char delim);
 
 bool is_positive_num(const std::string& str, bool hex);
 
+bool endsWith(const std::string& str, const std::string& suffix);
+
 #define INBOUNDS(index, vector) ((int) index >= 0 && (int) index < (int) vector.size())