Loading custom metainfos updates the global metainfo (through the `_allInheritingSections`)

Now, we use a member variable _allInheritingSections in each definition trying to keep track of inheriting sections, but the implementation leads to unwanted side effects. Loading (or reloading) custom metainfos updates the global metainfo, as they permanently add themselves to the _allInheritingSections list. Also, the _qualifiedName and _section attributes are incorrectly overwritten when inheriting sections are encountered.