This commit turns the base struct `uiItem` and all of its descendants
(including `uiButtonItem`, uiLayout`, etc.) into a C++ polymorphic
hierarchy of types.
This allows to use C++ type of memory management, and use non-trivial
types (which will be required to make `PointerRNA` non-trivial).
It also moves the storage of these `uiItems` from `BLI_listbase` to
`blender::Vector`, as our C-based listbase implementation is
incompatible with C++ polymorphism.
This also lead to making `uiItem` parameters of a few utils functions
`const`, to allow passing around `blender::Span` instead of vectors to
some internal helpers.
Pull Request: https://projects.blender.org/blender/blender/pulls/124405
a3f0d81a5e