MSVC: Embed debug visualizer definitions in PDB

The MSVC linker allows embedding debugger visualization definitions (.natvis files) into output PDB's. Each Qt module could include its own visualizer file(s) to be embedded in the PDB generated by a debug build of that module. Debugging Qt applications in Visual Studio using these natvis-enhanced PDB's would then allow custom visualization of Qt types in the "Watch" window, instead of the default "raw" view. This would be possible just by linking to the Qt libraries. No additional extension/add-in would then be required.

(In what concerns the Qt Visual Studio Tools extension, having the .natvis files embedded in the Qt libraries eliminates the risk that the .natvis installed by the extension and the types in the Qt libraries are out of sync.)