Adds `showAddonsInMenu` in WebExtensionBrowserMenuBuilder to allow the option of removing `Add-ons`
item even if another extensions are displayed
Add unit tests to test if 'Add-ons' item visibility is updated properly

This new property will be a timestamp of the last time media started playing in
the current page or be 0 if media hadn't started playing or another page loaded
in this tab so media was stopped.

To observe the media changes and update this property LastMediaAccessMiddleware
will have to be set on BrowserStore.

...to allow the option of keeping `FLAG_SECURE` when calling `stop()`

loadWebsiteTest and loadWebsitesInMultipleTabsTest were failing because
AwesomeBarFeature was asking for a negative number of history suggestions.