Extract panel resize code from sidebar into Resizable element

Make resizable work in vertical axis
Make dock resizable
Have dock preserve size based on Anchor position
Make pane buttons work more correctly in pathological cases
Sync status bar dock button with dock visibility/anchor position

Co-Authored-By Mikayla Maki <mikayla@zed.dev>

crates/theme/src/theme.rs

@@ -48,7 +48,7 @@ pub struct Workspace {
     pub pane_divider: Border,
     pub leader_border_opacity: f32,
     pub leader_border_width: f32,
-    pub sidebar_resize_handle: ContainerStyle,
+    pub sidebar: Sidebar,
     pub status_bar: StatusBar,
     pub toolbar: Toolbar,
     pub disconnected_overlay: ContainedText,
@@ -152,6 +152,8 @@ pub struct Toolbar {
 
 #[derive(Clone, Deserialize, Default)]
 pub struct Dock {
+    pub initial_size_right: f32,
+    pub initial_size_bottom: f32,
     pub wash_color: Color,
     pub flex: f32,
     pub panel: ContainerStyle,
@@ -240,7 +242,9 @@ pub struct StatusBarLspStatus {
 
 #[derive(Deserialize, Default)]
 pub struct Sidebar {
-    pub resize_handle: ContainerStyle,
+    pub initial_size: f32,
+    #[serde(flatten)]
+    pub container: ContainerStyle,
 }
 
 #[derive(Clone, Copy, Deserialize, Default)]