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/editor.cpp

@@ -88,19 +88,6 @@ bool compare_nocase (std::string first, std::string second)
         return false;
 }
 
-static bool endsWith(const std::string& str, const std::string& suffix)
-{
-    if (str.size() < suffix.size())
-    {
-        return false;
-    }
-    return str.compare(
-        str.size() - suffix.size(),
-        suffix.size(),
-        suffix
-    ) == 0;
-}
-
 void editorclass::loadZips()
 {
     directoryList = FILESYSTEM_getLevelDirFileNames();