We used to detect the `rcc` features before every `rcc` list invocation
wich resulted in `rcc` be called twice for every listing operation.
Now we detect the `rcc` list capabilities once during configuration and
pass it to the cmake_autorcc target in the info file.
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.
