This patch reworks the view-ID handling within the nitpicker GUI server
and the window manager. The namespace of view handles are now represented
as an Id_space. In constrast to the former "handles", which could be
invalid, IDs cannot be semantically overloaded with anything other than
an actual view reference. There is no notion of an invalid handle.
IDs are like C++ references (which cannot be a nullptr).
This change requires the code to be more explicit. E.g., the stacking of
a few at the front-most position can no longer be expressed by passing
an invalid handle as neighbor.
Issue #5242
d02a3d25d0