Make identifier generation more resilient to "non-valid" characters in keys

[Yeah69]

ResX keys may contain characters which aren't valid for property and field identifiers (e.g. spaces or parentheses). Make the generator more resilient. By …

• either replacing non-valid characters with a valid or no character
  • with this approach collisions have to be checked and resolved
• or throw an compile time exception which expresses the issue
  • currently in such cases the error messages are misleading