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.

@@ -146,3 +146,3 @@
   bContextStore *context;
   uiLayout *parent;
-  ListBase items;
+  blender::Vector<uiItem *> items;

@@ -375,3 +366,3 @@
 {
   if (item->type == ITEM_BUTTON) {
-    uiButtonItem *bitem = (uiButtonItem *)item;
+    const uiButtonItem *bitem = dynamic_cast<const uiButtonItem *>(item);