Remove Strings/AnsiStrings unit dependency for string handling

[Free-Pascal-meets-SDL-Website]

See PR #109 and discussion there.

In short:

• replace the Strings.StrLen (FPC) / AnsiStrings.StrLen (Delphi) function calls by SDL_strlen() function calls
• remove dependency from Strings (FPC) / AnsiStrings (Delphi) unit