Add validation to entry editor

[tobiasdiez]

I just remembered that my initial motivation to convert the entry editor to JavaFX came from my desire to display the integrity check messages directly in the entry editor. This PR accomplishes exactly this.

The location and style of the tooltip is not perfect but as of now these things cannot be changed since they are hardcoded in controlsfx (will create a feature request).

What do you think, should this still merged before the beta3 release? In the end it is only a minor change and shouldn't break anything.

• Change in CHANGELOG.md described
• Tests created for changes
• Screenshots added (for bigger UI changes)
• Manually tested changed features in running JabRef
• Check documentation status (Issue created for outdated help page at help.jabref.org?)
• If you changed the localization: Did you run gradle localizationUpdate?

[Siedlerchr]

codewise lgtm, but have not tested iit

[lenhard]

I have tested it locally and it looks good. I don't think it's a problem to merge it even into beta3, since it is a minor change and no new feature.

I have one request for a usability optimization: the warning sign is very small and it seems that I have to locate the mouse on exactly one pixel so that the message appears. Do you see any chance of increasing the size of the warning sign or making it easier to display the warning message?

[tobiasdiez]

No sadly I didn't found a way to specify the size of the graphic. These things are all hard-coded in the controlsfx class (which contains the comment // TODO we shouldn't hardcode this - defer to CSS eventually). I created an issue and will revist the style once this is possible.