Fix key bindings in tooltips for ArdourButton buttons.

Fix key bindings in tooltips for ArdourButton buttons.

Sinced gtkmm2ext needs to get at the actions of widgets, I moved the
action to Gtkmm2ext::Activatable.  Not sure if the wisest thing to
do here would be to use Gtkmm::Activatable, but figured there's
a reason Paul didn't do so (the name set_related_action is from there
so presumably it's known about), so this is the simplest change
that allows access to the action in Gtkmm2ext.  The vfunc calling
stuff should probably move there as well...

git-svn-id: svn://localhost/ardour2/branches/3.0@10818 d708f5d6-7413-0410-9779-e7cbd77b26cf