find out some warnings and errors in widgets' i18n properties

This PR:

- warning for `i18n.getValueForKey`
- find out i18n properties which is existed but not in manifest
- find out inexistent i18n properties
- find out prop keys which is undefined in html or js files
- find out keys which is unset in i18n properties
- all above do not work with `ui/` but only `node_modules/oae-*/*/`