Saved groups, open groups in other windows, and open groups in the current window all have `command` handlers defined in the tab overflow menu (TOM) to perform an action when clicked. However, the tab groups spec shows additional actions that should be available when right-clicking on those tab groups in the TOM. This patch adds those context menus and wires up their functionality:
- opening in current window
- opening in, or moving to, a new window
- deleting an open group
- forgetting a saved group
Differential Revision: https://phabricator.services.mozilla.com/D233129
86ccb78df7
