* Move duplicated v0/v1 impl packages to v0 subpackages
Fixes#2616. The convention is adopted from the command API (v1)
whose impl classes have the v1 subpackage.
* Separate client code into its own packages everywhere
* Fix code style
* Fix code style, part 2
* Allow v0 in package names
* Allow v0 in package names (fixed)
(cherry picked from commit 9244241639)
* Fix item group being missing when Operator Tab is hidden
* Do not modify special item groups
* Mark impl as internal
* Select the modded tab after switching tabs
* Allow adding items to Operator Blocks
