Make individual dash panes reactive instead of making the dash Group actor reactive

Make individual dash panes reactive instead of making the dash Group actor reactive

The width of a Group actor ends up including the width of its hidden children,
so we were getting a reactive object as wide as the details pane that was
blocking the clicks to the workspaces underneath it even when the details
pane was actually hidden.

Not making the dash Group actor reactive solves this problem. However, we
have to make individual parts of the dash reactive instead so that the clicks
are not passed to the transparent actor underneath them. That transparent
actor is used for dismissing the additional panes when the user clicks over
the workspaces area.