Simplify resetting of the SecondaryToolbar Scroll/Spread mode buttons, and add a missing comment in PDFCursorTools

The names 'resetscrollmode'/'resetspreadmode' were probably *not* great choices, given that the only thing being reset are toolbar buttons and not the actual Scroll/Spread modes. Furthermore, there's really no need for two separate events here.

The patch also adds a comment that ought to have been included in PR 9040, to prevent future refactoring/removing of what may appear to be an unnecessary `Promise.resolve` call.

web/pdf_cursor_tools.js

@@ -47,6 +47,8 @@ class PDFCursorTools {
 
     this._addEventListeners();
 
+    // Defer the initial `switchTool` call, to give other viewer components
+    // time to initialize *and* register 'cursortoolchanged' event listeners.
     Promise.resolve().then(() => {
       this.switchTool(cursorToolOnLoad);
     });