Refactor focusable/tabbalbe item registration toward allowing to handle it prior to clipping (not yet the case) (#343, #4079)

Now performed in ItemAdd(). It can't be trivially moved above clipping effectively because it would require us to scroll to be useful, meaning we'd be better off locking the bounding box a frame earlier. Still wip.
As-is this commit has no value for end-user, but it's a reengineering that moves us closer to the solution. + Internals: moved internal flags.
4 files changed