Move AIAlert* into namespace AIAlert and rename classes.

Instead of having several AI* classes, it turned out to be easier to
have a namespace: this allows me to define the classes in llcommon but
add (new) 'add' and 'add_modal' functions to 'AIAlert' in llui.
This is needed to avoid a collision with the 'add' functions in
LLNotificationsUtil.

The new add/add_modal makes it a lot easier to just show a caught
alert, prepending or appending new text: it turns out that that is
way more common then a re-throw.

Adjusted code as appropriate.