Add-on store: Use close button instead of OK/Cancel (#14854)
Part of #13985
Summary of the issue:
The add-on store makes more sense with a "Close" button instead of "OK/Cancel".
When exiting the dialog, users should be prompted to restart if they have changed the status of any add-ons.
Description of user facing changes
The "OK/Cancel" buttons have been replaced by a single "Close" button.
Pressing enter no longer closes the dialog, but "escape" does.
Description of development approach
The SettingsDialog now accepts "buttons" as a parameter. A developer can specify a set of buttons including OK, Cancel, Apply, Close.
