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

Cargo.lock

@@ -5412,7 +5412,6 @@ dependencies = [
  "multi_buffer",
  "panel",
  "picker",
- "popover_button",
  "postage",
  "project",
  "schemars",
@@ -7047,7 +7046,6 @@ dependencies = [
  "language_model",
  "log",
  "picker",
- "popover_button",
  "proto",
  "ui",
  "workspace",
@@ -10012,14 +10010,6 @@ version = "0.2.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5da3b0203fd7ee5720aa0b5e790b591aa5d3f41c3ed2c34a3a393382198af2f7"
 
-[[package]]
-name = "popover_button"
-version = "0.1.0"
-dependencies = [
- "gpui",
- "ui",
-]
-
 [[package]]
 name = "postage"
 version = "0.5.0"