Set tab-index="-1" for todo checkboxes

Otherwise they will be focusable when opening the search panel and pressing tab to cycle between the fields

src/editor/todo-checkbox.ts

@@ -20,6 +20,7 @@ class CheckboxWidget extends WidgetType {
         let box = document.createElement("input")
         box.type = "checkbox"
         box.checked = this.checked
+        box.tabIndex = -1
         box.style.margin = "0"
         box.style.padding = "0"