Bug 1935482 - Replace legacy properties with SidebarState properties r=sidebar-reviewers,sessionstore-reviewers,nsharpley,sfoster

- Move resize observer (setting max width) logic over to SidebarState.
- Add `command` property to SidebarState.
- Handle receiving legacy session restore data.
- Add unit tests along with mocking structures to get a better idea of state/controller coupling (which we should ultimately reduce).

Differential Revision: https://phabricator.services.mozilla.com/D232730

browser/components/sessionstore/SessionStore.sys.mjs

@@ -5122,22 +5122,8 @@ var SessionStoreInternal = {
       delete winData.hidden;
     }
 
-    let sidebarBox = aWindow.document.getElementById("sidebar-box");
-    let command = sidebarBox.getAttribute("sidebarcommand");
-    winData.sidebar = {};
-    if (sidebarBox.style.width) {
-      winData.sidebar.width = sidebarBox.style.width;
-    }
-    if (command && sidebarBox.getAttribute("checked") == "true") {
-      winData.sidebar.command = command;
-    } else if (winData.sidebar?.command) {
-      delete winData.sidebar.command;
-    }
-
-    if (aWindow.SidebarController.revampComponentsLoaded) {
-      const sidebarUIState = aWindow.SidebarController.getUIState();
-      winData.sidebar = structuredClone(sidebarUIState);
-    }
+    const sidebarUIState = aWindow.SidebarController.getUIState();
+    winData.sidebar = structuredClone(sidebarUIState);
 
     let workspaceID = aWindow.getWorkspaceID();
     if (workspaceID) {
@@ -6225,7 +6211,7 @@ var SessionStoreInternal = {
     if (!aSidebar || isPopup) {
       return;
     }
-    aWindow.SidebarController.setUIState(aSidebar);
+    aWindow.SidebarController.initializeUIState(aSidebar);
   },
 
   /**