Move PopoverButton into ui (#25724)

This PR moves the `PopoverButton` component into the `ui` crate.

The `popover_button` crate only depended on `ui`, so there doesn't seem
to be a need for it to live in its own crate and add another step in the
crate graph.

Release Notes:

- N/A

crates/ui/src/components.rs

@@ -19,6 +19,7 @@ mod modal;
 mod navigable;
 mod numeric_stepper;
 mod popover;
+mod popover_button;
 mod popover_menu;
 mod radio;
 mod right_click_menu;
@@ -56,6 +57,7 @@ pub use modal::*;
 pub use navigable::*;
 pub use numeric_stepper::*;
 pub use popover::*;
+pub use popover_button::*;
 pub use popover_menu::*;
 pub use radio::*;
 pub use right_click_menu::*;