Feature history on the left side, with contiguous folders (#449)

The feature history can sit in a panel on the left, with folders that hold one unbroken run of features, drag and drop as one undo step, and a resizable divider. Timeline folders save under format 17.

crates/ui-egui/src/lib.rs

@@ -4,7 +4,7 @@
 //! commands ([`SolveApp::run`]). Nothing here owns design data. The layout follows the familiar
 //! parametric-CAD desktop: application bar, a toolbar of workspace tabs and panels, the browser
 //! on the left, the 3D viewport with an axis view and navigation bar, the command dialog on the
-//! right of the viewport, the command palette and the timeline at the bottom.
+//! right of the viewport, the command palette and a bottom or left-sidebar feature timeline.
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 #![forbid(unsafe_code)]
 
@@ -117,6 +117,7 @@ pub mod sketch_tools;
 pub mod stand_up;
 pub mod theme;
 pub mod timeline;
+pub mod timeline_left;
 pub mod titlebar;
 pub mod toolbar;
 pub mod toolbar_custom;
@@ -1077,12 +1078,19 @@ impl SolveApp {
             }
         }
         toolbar::toolbar(self, ui);
-        if self.ui.show_timeline {
+        let left_timeline = self.ui.show_timeline && self.preferences.timeline_left;
+        if self.ui.show_timeline && !left_timeline {
             timeline::timeline(self, ui);
         }
         palette::palette_bar(self, ui);
         if self.ui.show_browser {
-            browser::browser(self, ui);
+            if left_timeline {
+                browser::browser_below_timeline(self, ui);
+            } else {
+                browser::browser(self, ui);
+            }
+        } else if left_timeline {
+            timeline_left::left_panel(self, ui);
         }
         preview::update(self, ui.ctx());
         egui::CentralPanel::default().frame(egui::Frame::NONE).show(ui, |ui| {