save settings whenever menu items are changed

This is to fix for #2602.

We connect SIGNAL MyAvatar::transformChanged() to SLOT Application::bumpSettings() which increments a number.  Similarly we connect the SIGNAL changed() of each checkable menu item to the SLOT bumpSettings(). Finally, we manually call bumpSettings() when identity info changes.  Every several ms or so we check for changes in that number and save settings if we find any.