WIP: Resizing splits (#2715)

We're finally doing the thing. 


TODO:
- [x] Choose an approach 
- Decided to add a new element just for the pane axis, containing a
slimmed down copy of the flex code.
- [x] Wire through callbacks and pointers so that data goes where it
needs to
- [x] Do the flex juggling math on resize
- [x] Update the flexes when updating the split tree
- [x] Restore the active_pane_magnification setting
- [x] Serialize an axis' flexes

Release Notes:
- Made the center pane group splits resizable. Note that resizing is
disabled if the `active_pane_magnification` setting is changed from
default.

crates/editor/src/element.rs

@@ -1182,8 +1182,10 @@ impl EditorElement {
         });
         scene.push_mouse_region(
             MouseRegion::new::<ScrollbarMouseHandlers>(cx.view_id(), cx.view_id(), track_bounds)
-                .on_move(move |_, editor: &mut Editor, cx| {
-                    editor.scroll_manager.show_scrollbar(cx);
+                .on_move(move |event, editor: &mut Editor, cx| {
+                    if event.pressed_button.is_none() {
+                        editor.scroll_manager.show_scrollbar(cx);
+                    }
                 })
                 .on_down(MouseButton::Left, {
                     let row_range = row_range.clone();