Convert Components.ConversationActions to a widget that encapsulates
selectively visible conversations action groups. Use multiple instances
of that in preference to a single shared instance that gets re-parented
as the responsive UI adjusts.
Flatten the custom widget hierarchy a bit by merging the
`ConversationActionBar` and `ConversationHeaderbar` widgets into
`MainToolbar` itself.
This creates a new object that contain the 4 groups of actions that used
to be in the conversation-viewer headerbar.
This allows the widgets to be moved to differen locations, e.g. to an
action bar that will be added in a later commit.
