Session restore code doesn't use the standard Tabbrowser.addTabGroup API, so tab groups that enter the DOM via session restore don't raise any TabGroupCreate events. To be consistent, and in order to support addons that need to have an up-to-date view of the tab strip, this patch fires TabGroupCreate on each tab group that enters the DOM.
This patch renames the existing `TabGroupCreate` fired from Tabbrowser.addTabGroup to `TabGroupCreateByUser`. This new event will fire after the tab group enters the DOM (therefore after `TabGroupCreate`) in scenarios where code calls Tabbrowser.addTabGroup with `isUserTriggered = true` to indicate that a user took an explicit action to create this tab group as a new tab group.
Differential Revision: https://phabricator.services.mozilla.com/D246630
37cc744292