git_ui: Start unifying panel style with other panels (#24296)

- Adds the `panel` crate for defining UI shared between panels, like
common button and header designs, etc
- Starts to update the git ui to be more consistent with other panels

Release Notes:

- N/A

crates/workspace/src/workspace.rs

@@ -21,7 +21,8 @@ use client::{
 };
 use collections::{hash_map, HashMap, HashSet};
 use derive_more::{Deref, DerefMut};
-use dock::{Dock, DockPosition, Panel, PanelButtons, PanelHandle, RESIZE_HANDLE_SIZE};
+pub use dock::Panel;
+use dock::{Dock, DockPosition, PanelButtons, PanelHandle, RESIZE_HANDLE_SIZE};
 use futures::{
     channel::{
         mpsc::{self, UnboundedReceiver, UnboundedSender},