Currently most of the data stored in `wmWindowManager` is runtime
data not saved to files. It's confusing that it's declared in DNA then. That
also prevents us from using C++ features. This commit adds an initial
runtime struct. Moving data there can be done as a separate step.
Initially I wanted to look at moving the `ReportList` system to C++.
The runtime struct has to be defined in the blenkernel module because
the members are (will be) used there in a few places.
Pull Request: https://projects.blender.org/blender/blender/pulls/118157
21cea65ea6