no-icon when saving without custom icon fixed

When no custom icon was loaded for the container and the scene file is
saved, the container would not have any icon displayed at all. This was
fixed in `ContainerObject::Read()`

Minor typos in the code description have been fixed.

`ContainerObject::OnDescriptionCommand()` was cleaned up by introducing
a new function `ProcessLevel()` which hides/unhides a hierarchical
level as required for hiding/unhidung tags or objects.

The global functions `ChangeNBitRow()` and `CountNBits()` have been
turned into static functions.