and their related name.
Also move the component module in the item_Tree module
register_component -> register_item_tree
ComponentItemTree -> ItemTreeNodeArray
For #3323
Rationale from that issue:
Right now, we use the term `component` in the Slint language and in the compiler to be a a tree of elements that can be used as an element in a .slint file.
The term is also currently used in the runtime as a tree of runtime Items that are allocated together. (declared in ComponentVTable)
But there are no 1 to 1 mapping between a `component` in the slint language, and a runtime Component.
(for example, the items behind a `if` or `for` or `PopupMenu` are in a different runtime component. And `component` declared in Slint are often inlined)
So we should rename the internal runtime `Component` to `ItemTree`
The currently public `slint::ComponentHandle` wraps the "root" of the ItemTree, but that's ok because it is generated from a .slint `component`, so it doesn't change name
