UI: Add initial "grid view"

Part of T98560.
See https://wiki.blender.org/wiki/Source/Interface/Views

Adds all the basic functionality needed for grid views. They display
items in a grid of rows and columns, typically with a preview image and
a label underneath. Think of the main region in the Asset Browser.

Current features:
- Active item
- Notifier listening (also added this to the tree view)
- Performance: Skip adding buttons that are not scrolled into view
  (solves performance problems for big asset libraries, for example).
- Custom item size
- Preview items (items that draw a preview with a label underneath)
- Margins between items scale so the entire region width is filled with
  column, rather than leaving a big empty block at the right if there's
  not enough space for another column (like the File and current Asset
Browser does it).
- "Data-View Item" theme colors. Not shown in the UI yet.

No user visible changes expected since the grid views aren't used for
anything yet.

This was developed as part of a rewrite of the Asset Browser UI
(`asset-browser-grid-view` branch), see T95653. There's no reason to
keep this part in a branch, continuing development in master makes
things easier.

Grid and tree views have a lot of very similar code, so I'm planning to
unify them to a degree. I kept things separate for the start to first
find out how much and what exactly makes sense to override.

source/blender/editors/interface/interface_intern.h

@@ -351,6 +351,13 @@ typedef struct uiButTreeRow {
   int indentation;
 } uiButTreeRow;
 
+/** Derived struct for #UI_BTYPE_GRID_TILE. */
+typedef struct uiButGridTile {
+  uiBut but;
+
+  uiGridViewItemHandle *view_item;
+} uiButGridTile;
+
 /** Derived struct for #UI_BTYPE_HSVCUBE. */
 typedef struct uiButHSVCube {
   uiBut but;
@@ -1365,6 +1372,7 @@ void ui_but_anim_decorate_update_from_flag(uiButDecorator *but);
 bool ui_but_is_editable(const uiBut *but) ATTR_WARN_UNUSED_RESULT;
 bool ui_but_is_editable_as_text(const uiBut *but) ATTR_WARN_UNUSED_RESULT;
 bool ui_but_is_toggle(const uiBut *but) ATTR_WARN_UNUSED_RESULT;
+bool ui_but_is_view_item(const uiBut *but) ATTR_WARN_UNUSED_RESULT;
 /**
  * Can we mouse over the button or is it hidden/disabled/layout.
  * \note ctrl is kind of a hack currently,
@@ -1396,6 +1404,8 @@ uiBut *ui_list_row_find_mouse_over(const struct ARegion *region, const int xy[2]
 uiBut *ui_list_row_find_from_index(const struct ARegion *region,
                                    int index,
                                    uiBut *listbox) ATTR_WARN_UNUSED_RESULT;
+uiBut *ui_view_item_find_mouse_over(const struct ARegion *region, const int xy[2])
+    ATTR_NONNULL(1, 2);
 uiBut *ui_tree_row_find_mouse_over(const struct ARegion *region, const int xy[2])
     ATTR_NONNULL(1, 2);
 uiBut *ui_tree_row_find_active(const struct ARegion *region);
@@ -1533,8 +1543,10 @@ void ui_interface_tag_script_reload_queries(void);
 /* interface_view.cc */
 
 void ui_block_free_views(struct uiBlock *block);
-uiTreeViewHandle *ui_block_view_find_matching_in_old_block(const uiBlock *new_block,
-                                                           const uiTreeViewHandle *new_view);
+uiTreeViewHandle *ui_block_tree_view_find_matching_in_old_block(const uiBlock *new_block,
+                                                                const uiTreeViewHandle *new_view);
+uiGridViewHandle *ui_block_grid_view_find_matching_in_old_block(
+    const uiBlock *new_block, const uiGridViewHandle *new_view_handle);
 uiButTreeRow *ui_block_view_find_treerow_in_old_block(const uiBlock *new_block,
                                                       const uiTreeViewItemHandle *new_item_handle);