Add $ref support for ComboBox + some optimization

[almirus]

code deduplication and add $ref support for ComboBox

[coveralls]

Coverage increased (+0.5%) to 91.134% when pulling 1a9a127 on almirus:master into 2959ba2 on JsonSchema-JavaUI:dev.

[SaifJerbi]

Good work to improve the code quality and remove code duplication

[SaifJerbi]

Hi @almirus,
Thanks for your contribution,
Could you please update the documentation and describe the new added property ?

[almirus]

I do not speak English well.
Examples for $ref:
https://github.com/json-schema-form/angular-schema-form/blob/bd182950b78c68ac3556b399fbaf3e3ea9a33232/examples/data/jsonref.json
json-schema-form/json-schema-form-core#1

It may be necessary to add support $ref for all fields

[SaifJerbi]

I found the combo box and the check box documentation aren't published yet, so i will add this property in the doc and publish that ASAP.
Don't hesitate if you find any improvement to do