(core) Improve focus and keyboard shortcuts in modals.

Summary:
- Factor out focusing logic from Clipboard to FocusLayer.
- Generalize FocusLayer to support adding a temporary layer while a modal is open.
- Stop Mousetrap shortcuts while a modal is open.
- Refactor how Mousetrap's custom stopCallback is implemented to avoid
  needing to bundle knockout for mousetrap.

Test Plan: Added a test that Enter in a UserManager doesn't open a cell editor from underneath the modal.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2626

app/common/RefCountMap.ts

@@ -49,6 +49,14 @@ export class RefCountMap<Key, Value> implements IDisposable {
     };
   }
 
+  /**
+   * Return the value for the key, if one is set, or undefined otherwise, without touching
+   * reference counts.
+   */
+  public get(key: Key): Value|undefined {
+    return this._map.get(key)?.value;
+  }
+
   /**
    * Purge a key by immediately removing it from the map. Disposing the remaining IRefCountSub
    * values will be no-ops.