Move dunecommon target to the root CMakeLists file

Summary

Introduce the dunecommon library target directly in the root CMakeLists.txt file.

The pattern I have introduced here is something that I think should be the default:

1. Create a module library target in the root CMakeLists.txt by dune_add_library(<target> [INTERFACE]) where INTERFACE should be used for header-only libraries
2. If there are sources to compile, add these to the library target in the corresponding dune/foo/sub/ directories, using target_sources(<target> PRIVATE <sources>...)
3. In case you want to set additional flags, or link against additional libraries, either do this in the CMakeLists.txt file you had in the lib/ subdirectory, or (better) directly in the root CMakeLists.txt file so that all target properties can be found in one spot.

It will be applied step by step also in other modules.