CMS-2762 Make ImageSelector set a valid name of embedded image content

Created api.Name class to ensure name validation. ContentName now extends api.Name
CMS-2762 Make ImageSelector set a valid name of embedded image content

Use fromString static method instead of constructor inside ensureValidName method
Merge pull request #510 from enonic/CMS-2710

CMS-2762 Make ImageSelector set a valid name of embedded image content