Add option to use ArduorButton as standalone Toggle/Checkbox.
Usually ArdourButton relegates this to Actions or is backed by Config.
This allows to use the Button as standalone Toggle, get_active()
It should maybe be a derived Class, but ArdourButton is already beyond
that..
The queued resize will only happen trigger a size-request when the
widget is realized, and on_size_request() calls ensure_layout().
Moreover, this over protection meant that sometimes a resize wasn't
noticed by the button containers.
