Add form validation

[b-strauss]

• send events for validation result/error
• autosend on submit (optional)
• multistep
• google recaptcha support
• multifile upload
• use FormData (with multiple values for key)
• native elements only
• state -> https://angular.io/guide/forms#track-control-state-and-validity-with-ngmodel
• non-ajax support (optional)
• reinitialize without losing state
• deactivate focus for non active parts
• file mimetypes in data attribute

[b-strauss]

• generic form groups with support for xor, all, min if present