UI: Expose an "is first search" boolean to search button callbacks

Currently when you open an RNA collection search button, like a
vertex group selector, the search filter isn't applied until you
start typing, in order to display every option at the start.
Otherwise they wouldn't be visible, since the search filter would
run for the current text.

Currently this check happens in one place, but it relies on the
`changed` value of `uiBut`. This is fine in the interface directory,
but anywhere else it would require exposing `uiBut.changed`, which
is probably too low-level to expose.

The solution is adding an `is_first` argument to the search callbacks,
which is nice for a few reasons:
  - They work at a higher level of abstraction, meaning they don't
    have to worry about how exactly to tell if this is the first
    search.
  - It makes it easier to do special behavior when the search menu
    is first opened.
  - Then, obviously, it makes that state accessible without including
    `interface_intern.h`.

Needed for attribute search: T85658

Differential Revision: https://developer.blender.org/D10528

source/blender/editors/interface/interface_utils.c

@@ -405,7 +405,8 @@ static bool add_collection_search_item(CollItemSearch *cis,
 void ui_rna_collection_search_update_fn(const struct bContext *C,
                                         void *arg,
                                         const char *str,
-                                        uiSearchItems *items)
+                                        uiSearchItems *items,
+                                        const bool is_first)
 {
   uiRNACollectionSearch *data = arg;
   const int flag = RNA_property_flag(data->target_prop);
@@ -415,7 +416,7 @@ void ui_rna_collection_search_update_fn(const struct bContext *C,
    * match the RNA name exactly. So only for pointer properties, the name can be modified to add
    * further UI hints. */
   const bool requires_exact_data_name = !is_ptr_target;
-  const bool skip_filter = data->search_but && !data->search_but->changed;
+  const bool skip_filter = is_first;
   char name_buf[UI_MAX_DRAW_STR];
   char *name;
   bool has_id_icon = false;