Initial asset shelf popup prototype

Plenty of work still, this crashes easily. Also needs some design
work. But it's something halfway working to evaluate, and it's quite fun
:)

@@ -0,0 +30,4 @@
+                                       const ScrArea *area,
+                                       AssetShelfType &shelf_type)
+{
+  ARegion *shelf_region = BKE_area_find_region_type(area, RGN_TYPE_ASSET_SHELF);

@@ -61,0 +68,4 @@
+/* -------------------------------------------------------------------- */
+/** \name Asset shelf type */
+
+bool asset_shelf_type_poll(const bContext &C,

@@ -61,0 +72,4 @@
+                           const SpaceType &space_type,
+                           const AssetShelfType *shelf_type);
+AssetShelfType *asset_shelf_type_find_from_idname(const SpaceType &space_type,
+                                                  blender::StringRefNull idname);

@@ -75,0 +75,4 @@
+                                                  StringRefNull idname)
+{
+  for (const std::unique_ptr<AssetShelfType> &shelf_type : space_type.asset_shelf_types) {
+    if (STREQ(idname.c_str(), shelf_type->idname)) {

@@ -0,0 +26,4 @@
+
+namespace blender::ed::asset::shelf {
+
+class PopupAssetShelfStorage {

@@ -0,0 +28,4 @@
+
+class PopupAssetShelfStorage {
+ public:
+  ListBase popup_shelves;