[UX] Make layouts aware of path aliases

If you attempt to create a menu entry for a node at /about, you can enter about into the menu UI, and it knows that's an alias. The menu entry it saves is actually for the system path, at node/2.

Could we do something similar for layouts? Many people who are new to backdrop try to put in an aliased path for the layout instead of the system path. Would it be possible check the path table, and do a switcheroo for them if we find a match?

For example, if they entered about we could * lookup the matching system path of node/2, then * realize that there's a system path of node/%, and * rewrite that field as node/% for them.

This might have to be done with ajax as soon as they tab out of that field, since the contexts in the next question need to be added dynamically too...