Add preference option to disable field validation

[lenhard]

In #3090 @tobiasdiez introduced field validation in the entry editor. This is clearly beneficial, but I can image that some users do not want it.

We should add a preference option that disables field validation for the entry editor.

[lenhard]

@tobiasdiez Could you please implement this one little addition to your validation feature. It is no big deal and we really need it for 4.0 (users are complaining about the validation).

[tobiasdiez]

I'll have a look next week but can't promise anything.

[tobiasdiez]

Sorry, I will not manage to do this in the foreseeable future.

[lenhard]

The option (default false) is now implemented and merged. The UI to change it will be added when the feature is fixed in controlsfx. Since it is enough to have one isse for tracking this, I'll close this one in favor of #3145.

Hence this can be removed from 4.0