Co-authored-by: Shi-Hao Hong <shihaohong@google.com>
Adds a disable argument in DropdownMenuItem widget and added tests.
Design Doc: docs.google.com/document/d/13W6PupVZUt6TenoE3NaTP9OCYsKBIYg9YgdurKe1XKs

This PR introduced a new class, ShortcutActivator, and refactored the the definition and lookup system of shortcuts, in order to solve a few issues of the previous algorithm.

This allows the key handling code in the shortcut manager to function when there is a null primaryFocus. The primary focus can be null only in some special circumstances (e.g. when an app hasn't set any focus at all), but that shouldn't cause a crash.

* Restorable FormField and TextFormField