Hand out an AccessKit focus request at the start of the pass

AccessKit asks for focus by node id, and `Focus::interested_in_focus` only picks
the request up once the widget itself runs. Everything that reads the focus
before the widget — styling it, for one — therefore saw the old focus, so a text
edit was styled unfocused in the very pass it got focused, and lost its focus
ring for a frame.

A widget in `focus_widgets_cache` has asked for focus in an earlier pass, so it
is one we can hand focus to in `begin_pass`, the same way a `request_focus` from
the last pass is handed out. Memory then holds the focus before any widget runs,
and `Ui::widget_style` needs to know nothing about focus at all.

`interested_in_focus` keeps the same grant as a fallback, for the first pass a
widget is focusable in, and the two share `take_focus_from_accesskit`.

The four `text_edit_*` snapshots gain a caret along with the ring, because the
field is now focused for real in that pass rather than a pass later.

Also drops `fallback_gap` from `TextEdit`, which has no `gap` of its own to defer
to, and moves the `DragValue` `HasClasses` impl above its test module.

tests/egui_tests/tests/snapshots/text_edit_delay_0_empty.png

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tests/egui_tests/tests/snapshots/visuals/drag_value.png

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