Both structure are bascially the same and only used internally, no need
to have the two.
The only difference was that one had the modifiers and the other one
not. But we can just set the modifier at the point we were converting
between the two.
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
