Move lib/frontend/XXX-adaptor.js to lib/frontend/api/XXX.js

There were two types in frontend, XXX-adaptor.js and XXX-handler.js.
I rename "adaptor" to "API" because REST is used as "REST API" not
"REST adaptor".

lib/frontend/XXX-adaptor.js ->
lib/frontend/api/XXX.js

lib/frontend/default-commands/ ->
lib/frontend/api/default-commands/

test/frontend/ ->
test/frontend/api/