Move most `uiLayout` and `uiItem` functions to a separate
`UI_interface_layout.hh` header file. `uiItem` and `uiLayout` structs
are moved from `interface_layout.cc` to `UI_interface_Layout.hh` in
preparation for switching to an object oriented API style.
`UI_block_layout*` functions are moved to `UI_interface_Layout.hh` too,
since their implementation is also in `interface_layout.cc`
Types shared by `UI_interface_Layout.hh` and `UI_interface_c.hh`
are moved to `UI_interface_types.hh`.
Ref: #117604
Pull Request: https://projects.blender.org/blender/blender/pulls/136717
7002f16992