Validator: relax parameter typehints

[xificurk]

• new feature
• BC break? no
• doc PR: not needed

Some validation methods of form Validator require a specifc type of control instead of generic IControl interface. This means you can't use those validators with custom form controls (e.g. any control based on nextras/form-components). This is especially frustrating for the basic "filled" validator which is used if the form control is marked as required.

This PR relaxes the parameter typehints in backward compatible manner to allow validation of custom controls.