Addon store: Implement updating add-ons (#14778)
Part of #13985
Summary of the issue:
An NVDA user may wish to update an add-on, either from the add-on store, or from an external source.
Description of user facing changes
Adds the ability to update an add-on, either from the add-on store or from an external source.
Description of development approach
Update method of determining update status.
