BKE_reports: make the API thread-safe.

This commit makes using (most of) `BKE_report` API safe in
multi-threaded situation.

This is achieved by adding a `std::mutex` lock to the `ReportList`
struct (in a slightly convoluted way unfortunately, due to this being a
DNA struct). This lock is then used to make most operations on
`Reportlist` data thread-safe.

Note that while working on this, a few other minor issues aroze in
existing usages of Reportlist by the WM code, mainly the fact that
`wm_init_reports` and `wm_free_reports` were both useless:
  - init was called in a context where there is not yet any WM, so it
    was doing nothing.
  - free was called on a WM that would be later freed (as part of Main
    freeing), which would also call cleanup code for its `reports` data.
Both have been removed.

Further more, `wm_add_default` (which is the only place where a WM ID is
created) did not initialize properly it reports data, this has been
fixed.

This change is related to the wmJob thread-safety tasks and PRs (#112537,
!113548).

Pull Request: https://projects.blender.org/blender/blender/pulls/113561

source/blender/python/intern/bpy_operator.cc

@@ -263,6 +263,7 @@ static PyObject *pyop_call(PyObject * /*self*/, PyObject *args)
 
       BKE_reports_clear(reports);
       if ((reports->flag & RPT_FREE) == 0) {
+        BKE_reports_free(reports);
         MEM_freeN(reports);
       }
       else {