Filter available field types

[PerryRylance]

Is your feature request related to a problem? Please describe.
I need a form builder for a project in which my client will make customer facing forms, but I want to restrict the types of fields they can use. For example, they don't need a colour picker.

Describe the solution you'd like
Could we build on #275 and possibly have some mechanism like a filter callback, or overridable collectors?

Describe alternatives you've considered
I've had a really good look at the code and there doesn't seem to be any way to override this, there are no alternatives as far as I can ascertain.

[PerryRylance]

PR submitted!

[PerryRylance]

@Abdulmajeed-Jamaan I'm seeing the FQCN in the UI here after overriding the built in fields, looks like we'll need some mechanism here perhaps to pick the first of the available fields?

This happens because the string is hard coded here:

[PerryRylance]

See #425 for a proposed solution

[Abdulmajeed-Jamaan]

@Abdulmajeed-Jamaan I'm seeing the FQCN in the UI here after overriding the built in fields, looks like we'll need some mechanism here perhaps to pick the first of the available fields?

This happens because the string is hard coded here:

Great catch, Thanks @PerryRylance

Check this proposed solution #427 as well