Blender Git Loki

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