Add debugger extension support

[tonka3000]

f.e. QString is based on pimpl idom, so it is extremely hard to see the content of it by just visualize the members.

We should add debugger extension support via natvis files, because it is directly support by the C/C++ extension from Microsoft through the visualizerFile property in the launch.json file. The extension should the that property automatically.

The Visual Studio Qt tools use also natvis files, but the license (GPL) restriction make it impossible to use it directly, so writing our own one is the only good option here.