Fix focus handle leak (#26090)

This fixes a major performance issue in the current git beta.
This PR also removes the PopoverButton component, which was easy to
misuse.

Release Notes:

- Git Beta: Fix frame drops caused by opening the git panel

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Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/ui/src/components.rs

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