This is a large commit that serves multiple purposes

- Iterate source files only once and store all extracted
  information in a cmQtAutogenDigest class that can be reused.
  This is brings speed improvements because several properties
  are only evaluated once. More that that it helps to avoid
  duplication of code with non trivial files property checks.
- Fix the Visual Studio generator to use PRE_BUILD when possible.
- Convert `for( ... )` loops to C++11 range base loops where possible
  (cmQtAutogen*.cxx only).
- String concatenation optimizations.