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.c

@@ -6661,11 +6661,20 @@ void UI_but_func_search_set_tooltip(uiBut *but, uiButSearchTooltipFn tooltip_fn)
   but_search->item_tooltip_fn = tooltip_fn;
 }
 
+void UI_but_func_search_set_all_strings_valid(uiBut *but, const bool value)
+{
+  uiButSearch *but_search = (uiButSearch *)but;
+  BLI_assert(but->type == UI_BTYPE_SEARCH_MENU);
+
+  but_search->all_strings_valid = value;
+}
+
 /* Callbacks for operator search button. */
 static void operator_enum_search_update_fn(const struct bContext *C,
                                            void *but,
                                            const char *str,
-                                           uiSearchItems *items)
+                                           uiSearchItems *items,
+                                           const bool UNUSED(is_first))
 {
   wmOperatorType *ot = ((uiBut *)but)->optype;
   PropertyRNA *prop = ot->prop;